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CASSELL'S  REINFORCED    CONCRETE 


Cassell's 

• 

Reinforced   Concrete 

A  Complete  Treatise  on  the  Practice  and 

Theory  of  Modern  Construction 

in  Concrete-Steel 

Edited  by 

Bernard  E.  Jones 

Editor  of  "  Building  World  " 

Assisted  by 

Albert  Lakeman 

M.C.I.,   Honours  Medallist  in  Building  Construction 

and  by  a  Staff  of  Specialist  Writers 


Illustrated  by  171  Photographs  and  about  Five  Hundred 
Diagrams  and  Working  Drawings 


Cassell  and  Company,  Limited 
London,  New  York,  Toronto  and  Melbourne 

1913 


List  of  Chief  Contributors  and  Revisers 

FRANK  B.  GATEHOUSE,   F.C.S Portland  Cement 

THOMAS  POTTER,  M.G.I Concrete 

A.   B.   SEARLE Concrete,  Cement,  Waterproofing,  etc. 

E.  L.  RHEAD,  M.Sc.,  Tech.  F.I.G Steel 

ALBERT  LAKEMAN,  M.G.I Theory,  Examples,  etc. 

Honours  Medallist  in  Construction 

V.   SUSSEX  HYDE,   M.G.I.       .     Practical  Erection,  Forms  and  Centerings 

F.  CHARLES  BARKER,  M.S.A Examples  of  Forms 

Assist.  Dangerous  Structure  Surveyor,  City  of  London 

T.  ELSON  HARDY Architectural  Treatment 

A.  SEYMOUR  JENNINGS,  F.I.B.D.          .       .        ...      Painting  Concrete 

W.   H.   BROWN,   F.S.I Estimating,  Quantities,  etc. 

THE   EDITOR  ....     Introduction,  History,  Examples,  etc. 


267323 


Preface 

THE  purpose  of  this  book  is  to  provide  a  practical,  simply-worded  guide  to  construc- 
tion in  reinforced  concrete,  as  distinct  from  the  theoretical  and  often  obscure  treatises 
that  have  appeared  in  large  numbers  during  the  past  few  years.  The  theoretical  side 
of  the  subject  has  not,  however,  been  overlooked,  as  any  treatise  on  reinforced  con- 
crete that  ignored  methods  of  calculating  the  various  members  would  be  obviously 
incomplete,  especially  as  this  method  of  construction  offers  great  possibilities  of 
scientific  design.  As  a  matter  of  fact,  the  two  theoretical  chapters  are  a  strong  feature 
of  the  book,  having  been  written,  at  my  request,  by  Mr.  Albert  Lakeman,  who  has 
brought  a  trained  mind  to  bear  upon  his  subject,  and,  in  addition  to  explaining  impor- 
tant principles  in  a  simple  and  concise  manner  and  clearly  showing  the  practical 
application  of  the  theories  he  expounds,  has  imparted  to  his  text  the  unusual  element 
of  freshness.  Mr.  Lakeman  has  wisely  begun  at  the  beginning,  and,  whilst  he  could 
not  finish  at  the  end — for  one  thing,  the  end  is  not  yet — he  will  be  found  to  take  an 
apt  and  diligent  student  far  enough  on  the  road  for  him  to  be  able  to  find  the  rest 
of  the  way  for  himself ;  and,  after  all,  no  book  can  do  more  than  that  for  anyone. 

The  book  is  largely  concerned  with  the  practical  side  of  reinforced  concrete — 
with  what  has  been  done  and  can  be  done  in  the  compound  material,  and  how  to  do 
it.  The  introductory  chapter  having  explained  the  advantages  of  reinforced  con- 
crete, an  historical  chapter  (every  definite  date  in  which  I  have  verified  from  official 
records)  puts  the  reader  in  possession  of  a  variety  of  facts,  many  of  which  will  throw 
light  on  the  subject  as  his  knowledge  of  it  grows.  Concrete  and  steel,  as  materials, 
are  discussed  in  the  two  following  chapters,  in  the  earlier  of  which  special  attention 
is  devoted  to  portland  cement  and  the  modern  methods  of  mixing  and  handling 
concrete.  Then  come  the  explanations  of  theory,  followed  by  two  chapters  respec- 
tively devoted  to  the  methods  employed  in  erecting  a  building  and  the  forms  and 
centerings  necessitated  in  general  reinforced  concrete  work ;  these  two  chapters  are 
noteworthy,  I  think,  from  the  practical  character  of  the  information  they  give  and 
the  very  large  number  of  explanatory  drawings  and  photographs  which  they  include. 
Concise  descriptions  of  the  chief  commercial  systems  follow,  and  later  chapters  deal 
respectively  with  architectural  and  surface  treatment,  durability,  waterproofing, 
arches  and  bridges,  and  quantity  surveying,  estimating,  measuring  and  pricing.  The 
concluding  chapter  describes  and  illustrates  a  number  of  works  carried  out  in  re- 
inforced concrete,  varying  from  a  palatial  club-house  and  commercial  "  sky-scraper  " 
to  railway  sleepers  and  sewer  pipes. 

Every  endeavour  has  been  made  to  render  the  volume  as  complete  as  possible,  in 
the  hope  that  it  will  be  useful  to  all  classes  of  readers,  both  as  a  text-book  and  as  a 
work  of  reference. 

I  have  been  happy  in  having  the  co-operation  of  a  number  of  experts  (a  list  of 


viii  PREFACE 

whom  is  given  elsewhere).  Each  of  these  writers  has  contributed  or  revised  the  section 
relating  to  the  particular  phase  of  the  subject  he  is  most  competent  to  deal  with  ;  and 
from  many  of  them  I  have  received  valuable  suggestions  which  I  have  been  able  to 
apply.  I  wish  to  acknowledge,  also,  the  kindly  treatment  accorded  me  by  a  large 
number  of  firms  identified  with  special  systems,  materials,  machines,  etc.,  for,  in  a 
number  of  cases  they  have  supplied  me  with  information,  drawings,  and  photographs. 
Additional  thanks  are  due  to  Messrs.  Geo.  West  and  Co.  for  permission  to  make  brief 
extracts  from  the  historical  notes  published  in  a  "  Lock-woven  Mesh  "  handbook. 

I  owe  a  special  debt  of  gratitude  to  the  premier  concrete  journal,  Concrete  and  Con- 
structional Engineering,  which  has  placed  many  facilities  at  my  disposal  and  has 
been  good  enough  to  lend  me  a  large  number  of  blocks.  To  the  files  of  that  journal, 
too,  I  have  gone  for  help  in  many  instances  when  other  sources  of  information 
proved  vain. 

The  illustrations  to  this  work  include  no  less  than  171  photographs  and  about 
five  hundred  diagrams  and  working  drawings ;  and  the  large  number  of  "  progress  " 
photographs,  showing  the  actual  practice  of  reinforced  concrete  work,  constitute,  I 
hope,  a  specially  valuable  feature  of  the  book,  and  considerably  enhance  its  teaching 

value. 

B.  E.  J. 

La  Belle  Sauvage, 
London,  E.G. 


Contents 


PAGE 

Introduction :     What  Reinforced  Concrete  Is  1 

The  Limitations  of  Concrete — Reinforcement — Terminology — A  Compound 
Material — Comparisons. 

Historical  Notes  .        .        5 

Early  in  the  Nineteenth  Century — A  Concrete  Boat — Wilkinson — Frangois 
Coignet — Dennett,  Allen,  Ransome,  and  Scott — Joseph  Monier  Phillip 
Brannon — Thaddeus  Hyatt — Progress  between  1870  and  1892 — Edmond 
Coignet  and  Frangois  Hennebique — Progress  since  1892 — Notable  American 
Inventions. 

Concrete :    Materials,  Proportions  and  Mixing         .         .         .  12 

What  Concrete  Is — Natural  Aggregates- — Artificial  Aggregates — Size  of  Aggregate 
—Washing  Coarse  Aggregate — Sand — Portland  Cement — Natural  Cements 
— Slag  Cement — Lime  Concrete — Water  for  Concrete  Mixing — Proportioning 
Concrete — Gauge  Boxes  and  Measures — Mixing  Concrete  by  Hand — Machine 
Mixing  and  Mixers — Batch  Mixers  Described — Continuous  Mixers  Described 
—Testing  Efficiency  of  Mixing — Conveying  Concrete — Laying  Concrete. 

SteeJ  .         .      50 

Composition — Malleability — Ductility — Elasticity — Limit  of  Elasticity — Testing 
Steel — Specification — Modulus  of  Elasticity — Certificate  of  Tests — Fatigue 
—Toughness — Hardness — Bending  Tests — Shearing  Strength — Resistance  to 
Alternating  and  Repeated  Stresses — Effects  of  Heat — Co-efficient  of  Expan- 
sion— Other  Elements  Present  in  Steel — Methods  of  Manufacture. 

Stress  Simply  Explained  .  .       61 

Formulae — Principle    of    Moments — Levers — Reactions — Bending    Moments — 
Moment  of  Inertia — Section  Modulus  and  Moment  of  Resistance — Calculating 
Safe  Load  on  Beam — Columns  and  Struts — Shearing  Stress. 

The  Theory  of  Reinforced  Concrete  •  •       83 

Introductory — Data  for  Calculations — Beams  with  Single  Reinforcement — Calcu- 
lations for  Slabs — Beams  and  Slabs  with  Double  Reinforcement — Tee  Beams 
— Tee  Beams  Continuous  or  Fixed  at  Ends — Tee  Beams  with  Double  Rein- 
forcement-— Shearing  Stress  and  Adhesion — Columns — Columns  Eccentrically 
Loaded — Retaining  Walls. 

The  Erection  of  a  Reinforced  Concrete  Building     .  •     126 

Building  a  Factory — Tools  and  Appliances — Driven  Piles — "  Compressol  "  Con- 
crete Piles — The  Foundation  Slabs — The  Retaining  Wall — Columns — Walls — 
Staircases — Introducing  the  Reinforcements  into  Beam  Moulds— Forming 
Skeleton  Reinforcements — Making  a  Floor. 

Forms  and  Centerings 

Sizes  of  Timber — Striking  the  Forms — Setting  Out — Forms  for  Piles — Foundation 
Forms — Column,  Beam,  and  Slab  Forms — Column  Forms — Centerings  for 


x  CONTENTS 

PAGE 

Concrete  Floors  Carried  by  Steel  Joists — Wall  and  Partition  Forms — Imitating 
Masonry  Walls — Staircase  Forms — Metal  Forms  for  Walls,  Beams,  etc. — 
Forms  for  Cornices,  Mouldings,  etc. — Forms  for  Ornamental  Parapet — Forms 
for  Retaining  Walls — Forms  for  Silos — Forms  for  Domes — Forms  for  Tall 
Chimneys — Forms  and  Centering  for  Arches  and  Bridges — Suspended  Center- 
ing for  Bridges — Forms  for  Pipes  and  Sewers — Forms  for  Tanks — Form  for 
Rectangular  Reservoir — Forms  for  Fence  Posts. 

Systems  Described  .     215 

Armoured  Tubular  Flooring — Clinton  System — Coignet  System — Considere  System 
— Corr  Bar — Dentile  System — Expanded  Metal  System — Hennebique  System 
— Indented  Bar  System — Johnson's  Lattice  System — Kahn  System — Keedon 
System — Lock-woven  Mesh  System — Mushroom  System — Paragon  System — 
Piketty  System — Other  Systems. 

Architectural  and  Surface  Treatment  of  Reinforced  Concrete      .         .     229 

Architectural  Treatment — Surface  Treatment — The  Untouched  Surface — Brush 
Finish — Carborundum  Finish — Sand-blast  Finish — Bush-hammered  Finish — 
Facing  Concrete  in  the  Form — P.ebble  Dashing — Sand  Finish — Glazed  Finish — 
Plastered  Surfaces — Tiles,  Mosaic,  Sgraffito,  etc — Stuc  Work — Colouring  Con- 
crete— Body  Colours — Table  of  Pigments — Stains — Distempering — Painting. 

Durability  of  Reinforced    Concrete     •  •     25t> 

Mechanical  Destructive  Influences — Blows  and  Shocks — Earthquakes — Settle- 
ments— Frost — Fire — Chemical  Destructive  Influences — Atmosphere — Water 
— Sea  Water — Acids — Alkalies — Urine. 

Waterproofing  Concrete  •     266 

Superficial  Waterproofing — Mass  Waterproofing — Making  a  Non-porous  Concrete. 

Specifications,  Quantities,  Measuring,  Estimating,  and  Pricing 

The  Specialist  System — A  General  Specification — Quantities — General  Outline 
of  Bill  or  Schedule — Designs  Independent  of  the  Specialist — Measuring  Con- 
crete— Small  Holes — Measuring  Steel  Reinforcement — Measuring  Centering 
— "  Use  and  Waste  " — First-class  Centering — "  Centering  "  Clauses  in 
Preamble — Specially-designed  Centering — Openings — Centering  to  Walls — 
Moulds  for  Architectural  Features — Centering  to  Inclined  Slabs,  Beams, 
Columns,  etc. — Abstracting — Billing — The  Preamble — Billing  Measured  Items 
— Pricing — Prices  for  Concrete,  Steel  and  Centering. 

Arches  and  Bridges 

Design  of  Arches  and  Bridges — Examples  of  Arches  and  Bridges. 

Examples  •     311 

Royal  Automobile  Club's  Building — Royal  Liver  Building — Cathedrals  at  Manila 
and  Poti — Wesleyan  Hall,  Westminster — Free  Church,  Hampstead  Garden 
City — St.  Barnabas'  Church,  Jamaica — The  Majestic  Theatre,  Los  Angeles 
— National  Gallery  Extension — Aluminium  Works  at  Kinlochleven — Royal 
Insurance  Building,  London — Wall  with  Post  Buttresses — Retaining  Wall — 
Stairways — Chimney  Shafts — Pipes,  Sewers  and  Conduits — Wharf  at  Denne- 
mont — Jetty  Head  at  Thames  Haven — Lifeboat  Slipway- — Roofed  Reservoir 
— Spectators'  Stands,  etc. — Railway  Sleepers — Grain  Silos  at  Silvertown 
— Water  Tower  at  Singen. 


List    of   Illustrations 


FIG.  PAGE 

1  to  3. — Columns  of  Plain  Concrete,  Steel,  and  Reinforced  Concrete  respectively  :   Comparative 

Diagrams  .          .          .          .          .          .          .          .          .          .          .          .  3 

4  to  6. — Beams  of  Plain  Concrete,  Steel,  and  Reinforced  Concrete  respectively  :    Comparative 

Diagrams          ..............         3 

7. — Gauge  Box  with  Fixed  Handles       .          .          .          .          .          .          .          .          .          .          .28 

8. — Cement  Gauge         ..............       28 

9. — Sectional  Diagram  showing  Principle  of  Chicago  Cube  Mixer     .          .          .          .          .          .31 

10. — Chicago  Cube  Mixer,  with  Engine  and  Boiler  .........       31 

11. — "  Cut-away  "  View  of  Eclipse  Mixing  Drum     .........        32 

12. — Eclipse  Mixer,  with  Petrol  Motor     .          ...          .          .          .          .          .          .          .          .32 

13. — "  Cut-away "  View  of  Koehring  Mixing  Drum  .          .          .          .          .          .          .          .33 

14. — Koehring  Mixer,  with  Engine  and  Boiler  .........       33 

15. — Marsh-Capron  Non-tilting  Mixing  Drum   .          .  .          .          .          .          .          .          .33 

16. — "  Cut-away  "  View  of  Marsh-Capron  Tilting  Mixing  Drum          .          .          .          .          .          .34 

17. — Marsh-Capron  Mixer,  with  Drum  in  Tilting  Position  .......       34 

18. — McKelvey  "  Gravity  Shovel  " 35 

19  to  24. — Ransome  Mixers    .          .          .          .          .          .          .          .          .          .          .          .          .36 

25. — Scoops  in  Ransome  Mixing  Drum  (English)       .          .          .          .          .          .          ...          .36 

26. — Scoops  in  Ransome  Mixing  Drum  (American)  .          .          .          .  .          .          .          .36 

27. — Elevations  and  Plan  of  Ransome  Belt-driven  Mixer.          .......       37 

28. — Roll  Mixer,  with  Elevator  raised      .          .          . 37 

29. — Smith  Hand-driven  Mixer         .          .          .          .          .          .          .          .          .          .          .          .38 

30. — Smith  Power-driven  Mixer       ............       39 

31. — Victoria  Mixer,  with  Skip  Elevated  ..........       40 

32. — Victoria  Mixer,  with  Skip  Lowered  ..........       40 

33. — "  Cut-away  "  View  of  Victoria  Mixing  Drum    .          .          .          .          .          .          .          .          .       41 

34. — Express  Mixer         ..............       4i 

35. — Fawcett  Mixer         .  .............       41 

36. — Gaspary  Hand-driven  Tilting  Trough  Mixer  ........       42 

37. — Pansy  Mixer  ...............       42 

38. — Whalley  Mixer,  with  Engine  and  Boiler  ..........       43 

39. — Gaspary  Drum-type  Continuous  Mixer      ..........       43 

40. — Mason  Mixer  ..............       44 

41.— Bolte  Mixer ^  ....       44 

41a. — Hoppers  of  Bolte  Mixer          ............       44 

42.— Goltrin  Mixer 45 

43.— Coltrin  Mixing  Blades 45 

44. — Kent  Mixer    ...............       45 

45. — Perfect  Mixer          ..............       45 

46. — "  Cut-away  "  View,  showing  Principle  of  Trump  Mixer     .......       40 

47. — Ransome  Concrete  Cart  .............       46 

48. — Ransome  Concrete  Skip  or  Bucket  ..........       46 

49. — Ground  Plan,  Top  Plan,  and  Two  Elevations  of  Ransome  Tower.          .....       47 

50.— Graph  showing  Influence  of  Carbon  on  Tenacity  of  Steel          ....  .51 

51. — Graph  showing  Influence  of  Carbon  on  Ductility  of  Steel          ......       52 

52. — Test  Piece  before  and  after  Stretching 53 

53  to  55. — Photo-micrographs  of  Steel  .          .          .          .          .          .          •          •          •          •          58,  59 

56. — Lever  of  First  Order 62 

57. — Lever  of  Second  Order    .............       62 

58.— Lever  of  Third  Order 63 

59. — Diagram  Showing  how  a  Cantilever  Tends  to  Move.         .......       63 

60. — Triangular  Portion  of  Wall  Lifted  by  Movement  of  Cantilever.          .....       63 

xi 


LIST    OF    ILLUSTRATIONS 

^G-  PAGE 

61. — Beam  Loaded  Eccentrically     ........  .64 

62. — Diagram  Showing  how  Eccentrically  Loaded  Beam  Tends  to  Rotate          ....  64 

63. — Beam  Rotating  on  the  other  Abutment.          .....  .64 

64. — Beam  Carrying  Three  Concentrated  Loads        ...  65 

65. — Diagram  Illustrating  Clear  and  Effective  Spans         .....  65 

66. — Centrally-loaded  Beam     ......  65 

67. — Bending  Tendency  on  Beam    ........  66 

68. — Finding  Bending  Moment  of  Beam .          .          .  .          .          .          .          .          .          .66 

69. — Bending  Moment  Set  Up  to  Scale   .......  .67 

70. — Finding  Bending  Moment  of  Beam            ....  67 

71. — Lever  Arms  of  Reaction  and  Load           ..........  67 

72. — Beam  with  Two  Loads  Concentrated  at  Different  Points  .......  67 

73. — Cantilever,  with  Concentrated  Load  at  Outer  End    ........  68 

74. — Failure  of  Cantilever  due  to  Tension        ......  68 

75. — Failure  of  Cantilever  due  to  Compression          .........  68 

76. — Bending  Moment  in  Cantilever         .......  .68 

77. — Bending  Moment  of  Cantilever  Set  Out  to  Scale       ........  69 

78. — How  Centrally-loaded  Cantilever  Tends  to  Bend       ........  69 

79. — Cantilever  with  Two  Concentrated  Loads          .........  69 

80. — Bending  Tendency  in  Cantilever  with  Two  Concentrated  Loads          .....  70 

81. — Beam  with  Uniformly  Distributed  Load  ..........  70 

82. — Distributed  Load  acting  through  Centre  of  Gravity           .......  70 

83. — Bending  Moment  of  Beam  at  Intermediate  Point     ........  70 

84. — Bending  Moment  on  Beam  Set  up  to  Scale      .          .          .          .          .          .          .          .          .71 

85. — Beam  with  Concentrated  and  Distributed  Loading   .          .          .          .          .          .          .          .71 

86. — Cantilever  Carrying  Distributed  Load       ..........  71 

87. — Finding  Bending  Moment  at  Intermediate  Point  in  Cantilever  ......  72 

88. — Bending  Moment  in  Cantilever  Set  Out  to  Scale       ........  72 

89. — Cantilever,  with  Combined  Distributed  and  Concentrated  Loading     .....  72 

90. — Calculating  Moment  of  Inertia  of  Simple  Rectangular  Section  ......  78 

91  to  97. — Common  Sections  and  their  Moments  of  Inertia      .......  74 

98. — Calculating  Moment  of  Inertia  of  Rolled  Steel  Joist          .......  74 

99. — Calculating  Least  Moment  of  Inertia  of  Rolled  Steel  Joist         ......  75 

100. — Rotating  Tendency  of  Beam,  caused  by  Reaction  and  Weight           .....  75 

.101. — Forces  Acting  upon  One-half  of  Beam     ..........  75 

102. — Calculating  Section  Modulus  :    Beam  Centrally  Loaded      .......  75 

103. — How  Centrally  Loaded  Beam  Tends  to  Bend 76 

104  to  106. — Calculating  Section  Modulus  of  Beam   .........  76 

107. — Strut  under  Load 78 

108. — Diagram  Indicating  Stress  in  Part  of  Strut      .........  78 

109. — Beam,  under  Vertical  Shear,  assumed  to  Consist  of  Separate  Blocks          ....  79 

110. — Beam,  under  Horizontal  Shear,  assumed  to  Consist  of  Separate  Planks    ....  79 

111. — Shearing  Stress  in  Cantilever  which  Carries  Concentrated  Load  at  Outer  End       ...  80 

112. — Shearing  Stress  in  Cantilever  which  Carries  Uniformly  Distributed  Load      ....  80 

113. — Shearing  Stress  in  Cantilever  which  Carries  Uniformly  distributed   Load  and  Two  Concen- 
trated Loads 80 

114. — Shearing  Stress  in  Beam  which  Carries  a  Central  Concentrated  Load         ....  81 

115. — Shearing  Stress  in  Beam  which  Carries  Uniformly  Distributed  Load.          ....  82 
116. — Shearing  Stress  in  Beam  which  Carries  Uniformly  Distributed  Load  and  Three  Concentrated 

Loads 82 

117  and  118. — How  a  Single  Reinforced  Beam  Resists  Compression  and  Tension         ...  86 
119. — Diagram  showing  Proportionate  Stresses  above  and  below  Neutral  Axis  to  Produce  Deforma- 
tion            87 

120. — Finding  Position  of  Neutral  Axis     .                                                                                                  .  91 

121. — Section  through  Cross  Rods  .                                                                                                             .  99 

122. — Section  through  Longitudinal  Rods           ...  99 

123  and  124. — Double  Reinforcement  in  Beams          .                                                                              .  100 

125. — Designing  Beam  having  Double  Reinforcement          .                                                                    .  101 

126. — Section  of  Tee  Beam  and  Method  of  Finding  Total  Compression       ...                   .  104 

127.— Section  at  Centre  of  Span  of  Tee  Beam  fixed  at  Ends 110 


LIST    OF    ILLUSTRATIONS  xiii 

FIG.  PAGE 

128. — Section,  at  Ends  of  Span,  of  Tee  Beam  fixed  at  Ends     .         .         .         .         .          .          .111 

129. — Determining  Stress  in  Ordinary  and  in  Reinforced  Concrete  Beams  .         .         .          .         .113 

130. — Vertical  Shear  in  Reinforced  Concrete  Beam  carrying  Uniformly  Distributed  Load     .          .115 
131. — Horizontal  Shear  in  Reinforced  Concrete  Beam  carrying  Uniformly  Distributed  Load          .     116 
132. — Diagrams  showing  Need  for  Lateral  Reinforcement  in  Columns          .....     117 

133  and  134. — Rectangular  and  Circular  Columns  with  respectively  Rectilinear  and   Curvilinear 

Laterals  .          .          .          .          .          .          .          .          .          .          .          .          .          .          .118 

135. — Designing  Column  Eccentrically  Loaded  ..........     122 

136. — Stress  in  Column  Eccentrically  Loaded    ..........      122 

137. — •Influence  of  Angle  of  Repose  and  Line  of  Rupture  and  Design  of  Retaining  Walls   .          .     123 

138. — Three  General  Types  of  Retaining  Walls. 124 

139. — Retaining  Wall  with  Cantilevers  under  Footpath      ........     125 

140. — Retaining  Wall  to  Resist  Water  Pressure          .          .         .         .          .         .         .         .          .125 

141. — Vertical  Cross-section  of  Typical  Factory  Building  in  Reinforced  Concrete          .  .     126 

142.— Mixing  Stage 128 

143  to  146. — Four  Patterns  of  Iron  Tamper 128 

147. — Perforated  Spade .         .          .128 

148.— Ross  Spade 129 

149. — Tamper  for  producing  Fine  Surface  ..........     129 

150. — Special  Spade  for  Facing 129 

151. — Andrews  Tamper 129 

152.— Wooden  Tamper 129 

153. — Cutting  Anvil  and  Hammer 129 

154  and  155. — Wrenches  for  Bending  Ends  of  Bars  .........     130 

156  and  157. — Wrenches  for  Bending  Ends  of  Stirrups 130 

158. — Key,  or  Twister,  for  Bending  Ends  of  Stirrups,        .          .          .          .          .          .          .          .130 

159.— Curry  Tyer 130 

160. — Elevation  and  Plan  of  Bench  Bending  Machine         .          .          .          .          .          .         .          .     130 

161. — Kennedy  Bar-bending  Machine,  No.  1  .          .         .          .          .          .          .         .     •    .     130 

162. — Kennedy  Bar-bending  Machine,  Geared  Pattern         .          .          .          .         .         .         .          .130 

163. — Making  Bend  to  Given  Inside  Measurement     .........     131 

164. — Making  Bend  to  Given  Outside  Measurement  .         .          .          .          .          .         .         .         .131 

165.— Making  a  Double  Set 131 

166.— Making  Sharp  Bend  in  Thin  Bar .          .          .          .          .131 

167. — Photographic  View  of  Right  and  Left  Double  Bar-bender         ......     131 

168.— Method  of  Bending  Small  Bars 132 

169. — Hennebique  Square  Pile  ............     133 

170.— Coignet  Round  Pile 133 

171. — Considere  Octagonal  Pile 133 

172. — Lidgerwood  Pile-driving  Engine  and  Reinforced  Concrete  Pile  ......     134 

173.— Simplex  Pile  . 136 

174. — "Compressol"  Borer,  Rammer,  and  Tester 137 

175. — "  Compressol "  Frame  and  Borer  in  Use  .         .         .         .         .         .         ...         .     138 

176.—"  Compressol "  Pile 138 

177. — Plan  of  Typical  Pile  Caps  and  Connecting  Beams    ........     139 

178. — Section  showing  Pile  Caps,  Filling,  Beam,  and  Slab 139 

179. — Foundation  Slab  Reinforcement,  held  in  Notched  Templates     ......     140 

180. — Ra^t  Foundation  Beam  Reinforcement  on  Wooden  Supports     ......     140 

181. — Raft  Reinforcement,  Kingsway  Church,  London        ........     141 

182. — Completed  Raft,  Kingsway  Church,  London     .........     142 

183. — Plan  of  Retaining  Wall,  with  Tapering  Counterforts          .          .         .         .         .          .         .     142 

184. — Reinforcement  of  Retaining  Wall,  Royal  Insurance  Building      ......      143 

185  and  186. — Reinforced  Concrete  Retaining  Wall  for  Royal  Insurance  Building        .          .          .     144 
187. — Diagram  showing  the  above  Retaining  Wall  if  built  with  Brickwork          .         .         .         .     144 

188. — A  further  View  of  Retaining  Wall,  Royal  Insurance  Building,  in  Course  of  Erection  .  .  145 
189. — Cross  Sections  of  Columns  showing  Right  and  Wrong  Methods  of  Placing  Reinforcements  .  146 
190  and  191. — Horizontal  and  Vertical  Sections  through  Typical  Factory  Staircase  .  .  .  147 

192. — Hollowed-out  Concrete  Block  to  Facilitate  Spacing  of  Bars 147 

193.— Money  Order  Office,  Holloway,  showing  the  Centering  for  Pillars,  Floors,  and  Walls.  .  148 
194. — Money  Order  Office,  Holloway,  showing  Two  of  the  Wings  in  Course  of  Construction  .  149 


xiv  LIST  OF    ILLUSTRATIONS 

FIG.  PAGE 

195. — Method  of  Supporting  Reinforcement  in  Beam  Mould        .          .          .          .          .          .          .150 

196. — Typical  Floor  with  Continuous  Mesh  Reinforcement.          .......      151 

197. — Typical  Floor  with  Sheet  Mesh  Reinforcement  ........      152 

198.— Craig  Screw-bore 154 

199.— The  Wainwright  Steel  Kerb .154 

200.— Steps  with  Steel  Kerbs 154 

201. — Section  through  Column  with  Steel  Corner  Bars      .          .          .          .          .          .          .          .      154 

202.— Ebco  Corner  Bar 154 

203.— Setting  Out  Piers  of  Rectangular  Building .          .157 

204. — Horse  used  in  Setting  Out 157 

205. — Setting  Out  Acute  Angle  of  Building 157 

206.— Setting  Out  Obtuse  Angle  of  Building 157 

207.— Form  for  Square  Pile 158 

208.— Bolted  Form  for  Bound  Pile 158 

209.— Stayed  Form  for  Round  Pile  . 158 

210  and  211. — Elevation  and  Enlarged  Cross  Section  of  Pile-making  Platform    ....      159 

212. — Plan  and  Elevation  of  American  Pile-making  Forms  and  Platform    .          .          .          .          .159 

213. — Enlarged  Cross  Section  through  Form  shown  in  Fig.  212  ......      159 

214. — Concrete  Piles  and  Forms  Dissociated       ..........      159 

215.— Foundation  Form  Built  after  Slab  is  Hard 159 

216. — Forms  for  Pile  Caps,  Foundations,  etc.    .          .          .          .          .          .          .          .          .          .160 

217. — Typical  Form  for  Foundation 160 

218. — Cheaper  Type  of  Form  for  Foundations   .          .          .          ...          .          .          .          .          .161 

219.— Form  Strengthened  with  Wire 161 

220.— Typical  Column  Form,  with  Slid-in  Front  Boards 162 

221. — Typical  Column  Form,  with  Spiked-on  Front  Boards         .......      163 

222.— Clamped  Form  for  Short  Columns 164 

223.— Cheap  Type  of  Column  Form 165 

224. — Column  Form  with  Two  Sides  held  between  Fillets  and  Battens        .....     166 
225. — Column  Form  with  Two  Sides  held  between  Fillets  and  Battens        .....     167 

226.— Typical  Beam  Form .168 

227. — Boards  with  Splayed  Edges  to  allow  for  Expansion  .          .          .  .          .          .      168 

228. — Beam  Form  strutted  from  Extended  Base  Battens  ........      169 

229.— Folding  Wedges  under  Dead  Shore 169 

230. — Form  for  Two  Intersecting  Beams  ...........     170 

231. — Elevation  and  Cross  Section  of  Forms  for  Pillar  and  Beam  with  Splayed  Angles       .          .      171 
232. — Two  Cross  Sections  through  Hennebique  Floor  and  Plan  of  One  Bay        ....      172 

233.— Beam  Form  held  by  Clamp ....      172 

234. — Column  Form  with  Bolts  and  Thumbscrews     .........      172 

235  and  236. — Beam  Forms  used  in  Messrs.  Sainsbury's  Premises,  London          ....      172 

237. — Beam  and  Column  Forms  used  at  a  Bermondsey  Warehouse    .          .          .          .          .          .      173 

238.— Plan  of  Column  Form • 173 

239. — Plan  showing  Position  of  Clamps  for  Reduced  Column  Form    ......      173 

240. — Isometric  Sketch  of  Forms  for  a  Complete  Bay  of  a  Warehouse  Floor          ....      173 

241. — Early  Stage  of  Forms  and  Centerings  at  H.M.'s  Stationery  Office      .....      174 

242. — Beam  and  Column  Forms  used  at  H.M.'e  Stationery  Office       ......      175 

243  to  245. — Adjustable  Beam  Forms  designed  by  H.  Kempton  Dyson      .          .          .          .          .175 

246. — Centering  for  Beams  and  Floors  in  Storied  Building        .......      176 

247. — Form  for  Twelve-sided  Column 177 

248.— Form  for  Fluted  Column 177 

249. — Form  for  Diminished  Column  ............      177 

250  and  251. — Columns  at  Wesleyan  Hall,  Westminster     ........      178 

252. — Beam  Forms,  etc.,  at  Wesleyan  Hall,  Westminster  .          .          .          .          .          .          .          .      179 

253. — Form  for  Column  Base  at  Wesleyan  Hall,  Westminster    .......      180 

254. — Centering  for  Floor  at  Wesleyan  Hall,  Westminster  .......      180 

255. — Centering  Resting  on  Flanges  of  Steel  Joists    .          .          .          .          .          .          .          .          .181 

256. — Centering  Suspended  from  Flanges  of  Steel  Joists     .          .          .          .          .          .          .          .181 

257. — Centering  for  Concrete  Floor  having  Steel  Main  Beams     .          .          .          .          .          .          .181 

258. — Centering  for  Arch  Ceiling  between  Joist  Flanges     .          .          .          .          .          .          .          .181 

259  and  260. — Floor  Centering  supported  by  Hangers        ........      181 


LIST   OF    ILLUSTRATIONS  xv 

FI°-  PAGE 

261. — Form  for  Wall :    Part  raised  to  Second  Position       .          .          .          .          .          .          .          .  182 

262.— Section  of  Wall  Form 182 

263.— Panel  for  Wall  Form .182 

264  and  265. — Plan  and  Cross  Section  of  Shored-up  Form  for  Wall  ...                    .  183 

266. — Form  for  Simple  Foundation  Wall  ..........  184 

267. — Ransome's  Wall  Form     .          .          .          .          .          .          .          .'         .  184 

268.— Panel  Shutter .184 

269.— Collar  and  Set-screw  at  x  (Fig.  267)        .          . 184 

270. — Side  of  Form  for  Imitating  Masonry  Wall 185 

271. — Form  and  Centering  for  Staircase    ........                              .  185 

272. — Stringers  and  Carriage  of  Stairs        .........  185 

273.— Form  for  Steps .185 

274. — Metal  Panel  Form  for  Walls    ........  186 

275. — Method  of  fastening  Panel  Flanges  together      .          .          .          .          .          .          .          .          .  186 

276. — Detail  of  Metal  Beam  Form    ............  186 

277. — Ransome  Form  for  Cornice      .          ...          .          .          .          .          .          .          .          .          .  186 

278. — Form  for  Spandrel  Wall  to  Bridge  .          .          . .  186 

279. — Form  for  Curtain  Wall  with  Moulded  Cornice            ........  186 

280. — Two  Sections  of  Form  for  Cornice  to  Hollow  Coping         .......  187 

281. — Form  for  Ornamental  Parapet          ...........  187 

282  and  283. — Vertical  and  Horizontal  Sections  of  Form  for  Battered  Retaining  Wall         .         .  187 

284  and  285. — Plan  and  End  Elevation  of  Form  and  Centering  for  Retaining  Wall  at  Bridlington  188 

286. — Part  End  Elevation  and  Section  of  Centering  for  Retaining  Wall  at  Local  Government  Offices  189 

287. — Diagram,  Plan,  and  Details  of  Centering  for  Retaining  Wall  at  Local  Government  Offices  .  189 

288  and  289. — Plan  and  Section  of  Centering,  etc.,  for  Retaining  Wall  in  Deep  Basement   .          .  190 

290  and  291. — Elevation  and  Plan  of  Form  and  Centering  for  Silo    .          .          .          .          .          .  191 

292  and  293. — Vertical  Section  and  Plan  of  Centering  for  Dome  of  Wesleyan  Hall,  Westminster  192 

294. — Centering  for  Dome  at  Annapolis  New  Academy 193 

295  to  297. — Centering  for  Octagonal  Dome,  Circular  in  Section         ......  193 

298  and  299.— Reinforcing  the  Large  and  Small  Half  Cupolas  of  the  Poti  Cathedral       .          .          .194 

300  and  301.— Centerings  for  Belfry  and  Arches  of  the  Poti  Cathedral 195 

302  to  305. — Centerings  and  Reinforcement  for  Cupolas  and  Vaulting  at  the  Poti  Cathedral        .  196 

306.— Centering,  etc.,  for  Chimney  at  Northfleet 197 

307.— Centering  for  Flat  Bridge 198 

308. — Centering  for  Melan  Arched  Bridge.          .          .          .          ...          .          .          .         .  198 

309. — Elevation  of  Centering  for  Arched  Principal,  Hammersmith  Baths     .....  199 

310. — View  of  Centering  for  Arched  Principal,  Hammersmith  Baths.          ....  199 

311  and  312.— Centering  for  Arch  Ring  Bridge 200 

313. — Part  Elevation  and  Detail  of  Centering  for  Bridge  at  Teufen   ......  200 

314.— Detail  of  Bridge  Centering  (see  A,  Fig.  313)     .          . 201 

315.— View  of  Part  of  False-work  for  Bridge  at  Teufen 201 

316  and  317. — Centering  for  Almandares  Bridge,  Havana  .          .          .          .          .          .          .          .  201 

318.— Centering  for  Meadow  Street  Bridge,  Pittsburg 202 

319  and  320.— Centering  for  Bridge  of  233-ft.  Span 202 

321. — Centering  for  Bridge  over  River  Werra  in  Thuringia          .          .          .          .          .          .          .  203 

322.— Centering  for  Bridge  of  229-ft.  6-in.  Span 203 

323.— Centering  for  Small  Bridge •...''.          .          .          .203 

324.— Centering  for  Walnut  Lane  Bridge,  Philadelphia        . 204 

325.— Centering  for  Bridge  of  126-ft.  Span 204 

326.— Centering  for  Bridge  of  138-ft.  Span 204 

327.— Centering  for  Bridge  of  80-ft.  Span           .          . 205 

328.— Centering  for  Bridge  of  110-ft.  Span .          .          .         .205 

329  and  330.— Centering  for  Bridge  at  Deer  Park,  U.S.A.           . 205 

331.— Centering  for  Flat  Bridge  of  42-ft.  7-in.  Span 206 

332.— Centering  for  Flat  Bridge  of  88-ft.  6-in.  Span           .          . 206 

333.— One  of  the  Three  Spans  of  the  Chickahominy  River  Bridge 206 

334. — Elevation  of  Panel  of  Suspended  Centering       .........  206 

335. — Section  of  Panel  of  Suspended  Centering.          .........  206 

336. — Cables  supporting  Concrete  Sections,  showing  Key  Spaces  to  be  filled  in  .          .         .         .  207 

337.— Arch  Ribs  before  Striking  Suspended  Centering         .          . 207 


xvi  LIST    OF    ILLUSTRATIONS 

FIG.  PAGE 

338. — Arch  Bibs  formed  in  Suspended  Centering        .........     208 

339.— Centering  for  6-ft.  Sewer 208 

340.— Centering  for  8-ft.  6-in.  Sewer 208 

341. — Centering  for  Conduit  at  Jersey  City 209 

342.— Centering  for  5-ft.  Arched  Culvert 209 

343.— Centering  for  8-ft.  Arched  Culvert 209 

344.— Form  for  Small  Box  Culvert  .         . 209 

345.— Collapsible  Steel  Centering  for  Sewer 210 

346. — Conduit  at  Woolwich,  showing  Collapsible  Steel  Centering  in  Use      .....     210 

347  and  348.— Form  for  Square  Tank 211 

349  to  352.— Form  for  Circular  Tank 211 

353  and  354.— Form  and  Centering  for  Gasholder  Tank 212 

355.— Form  for  Gasholder  Tank  at  San  Sebastian 213 

356.— Gasholder  Tank  at  San  Sebastian 213 

357. — Elevation,  Plan,  and  End  View  of  Form  for  Tapered  Square  Posts  ...  .214 

358. — Cross  Section  (enlarged)  through  Form  for  Tapered  Square  Posts      .          .          .          .          .214 
359. — Plan  and  Elevation  of  Multiple  Form  for  Tapered  Square  Posts        ...  .     214 

360. — Section  of  Form  for  Triangular  Posts 214 

361  and  362.— Armoured  Tubular  Floor 215 

363.— Coignet  Beam  and  Slab 216 

364. — Coignet  Beam  Reinforcement  Consisting  of  Group  of  Small  Bars       .         .         .         .         .216 
365. — Beam  Supporting  Floor  Slab  Centering    ..........     216 

366. — Section  through  Coignet  Column '      t         .     216 

367  and  368.— Base  of  Coignet  Column 216 

369  and  370.— Coignet  Pipe  or  Conduit 216 

371.— Corr  Bar  Beam  "  Unit " .217 

372.— Types  of  Corr  Bars 217 

373.— Dentile  Floor  with  Mitre  Tiles .218 

374.— Dentile  Floor  with  Bridge  Tiles .218 

375.— Dentile  Floor  with  "  L "  Tiles .218 

376.— Diamond  Mesh  Expanded  Metal .218 

377.— Bib  Mesh  Expanded  Metal .218 

378.— Section  of  Expanded  Steel  Bar .     219 

379.— Four  Types  of  Expanded  Metal  Floors .  .     219 

380. — Hennebique  Stirrup  round  Tension  Bar .     220 

381. — Hennebique  Tension  Bars  and  Stirrups    ....  .     220 

382. — Simple  Hennebique  Beam •     220 

383  and  384. — Hennebique  Beams  Continuous  over  Intermittent  Supports  ....     220 

385. — Beam  Beinforcement  with  Compression  Bar  and  Double  Stirrups       .....     220 

386. — Hennebique  Column 220 

387  and  388. — Base  of  Hennebique  Column       .......  .     220 

389.— Sheet  Piles .220 

390.—  Hollow  Diaphragm  Pile .  .220 

391. — Square  Section  Indented  Bar 221 

392.— Bound  Section  Indented  Bar .  .221 

393.— Johnson's  Steel  Wire  Lattice  ....  .222 

394  to  397. — Four  Methods  of  Supporting  Mesh-reinforced  Floors 222 

398.— View  of  Kahn  Bar 223 

399. — Section  and  Elevation  of  Kahn  Trussed  Bar 223 

400  and  401.— Lintel  Beinforced  with  Kahn  Bar       ...  223 

402. — Keedon  Beam  Beinforcement  ...  .     223 

403. — Keedon  Column  Beinforcement         .  .     22& 

404. — Column  and^Beam  Beinforcements,  Keedon  System 223 

405. — Lock-woven  Mesh   ...  .     224 

406. — Lock- woven  Mesh  Floor  .          .  .     224 

407. — Fireproof  Construction  with  Lock- woven  Mesh.  .     224 

408. — Floor  Slab  Supported  by  Four  Columns,  Mushroom  System 224 

409. — Head  of  Column,  Mushroom  System         .  .     224 

410.— Paragon  Stirrups     .  .225 

411.— Paragon  Column,  Beam,  and  Floor  Slab .     225 


LIST   OF    ILLUSTRATIONS  xvii 

FIG.  PAGE 

412. — Paragon  Column  Hoopings       ............  £25 

413  and  414. — Paragon  Helical  Column  Wrappings    .........  225 

415. — Piketty  Beam           ..............  226 

416  and  417. — Piketty  Beams  with  Two  and  Three  Tension  Bars       ......  226 

418  and  419. — Cross  Sections  of  Piketty  Beam  with  Four  Rows  of  Bars    .....  226 

420  and  421. — Square  Piketty  Column  ...........  226 

422  and  423.— Round  Piketty  Column 226 

424. — The  Upper  Storeys  of  a  Reinforced  Concrete  Warehouse  at  Cologne,  Front  View  .  .  230 

425. — End  View  of  Reinforced  Concrete  Warehouse  at  Cologne 230 

426  to  428. — Vertical  and  Horizontal  Sections  of  Small  Hall  with  Arched  Ribs           .         .         .  231 
429. — Interior  View  of  Hall  with  Arched  Ribs            .         .         .          .         .         .         .         .         .231 

430  to  432. — Vertical  and  Horizontal  Sections  of  Small  Hall  with  Barrel  Roof  .         .                   .  232 

433. — Interior  View  of  Hall  with  Barrel  Roof  ..........  232 

434  to  436. — Vertical  and  Horizontal  Sections  of  Small  Hall  with  Modified  Barrel  Roof         .         .  233 

437  to  439. — Vertical  and  Horizontal  Sections  of  Small  Hall  with  Pierced  Arched  Ribs         .         .  234 

440. — Interior  View  of  Hall  with  Pierced  Arched  Roof       ........  234 

441. — Interior  View  of  Hall  with  Arched  Ribs  and  Vertical  Columns           .         .       •  .         .         .  235 

442. — Part  Longitudinal  Section  of  Hall  with  Arched  Ribs  and  Vertical  Columns         .         .         .  235 

443. — Cross  Section  of  Hall  with  Arched  Ribs  and  Vertical  Columns           .....  236 

444. — Part  Longitudinal  Section  of  Exhibition  Hall  with  Three  Flat  Saucer  Domes  .  .  .  237 

445.— Part  Plan  of  Exhibition  Hall  . 237 

446.— Interior  View  of  Exhibition  Hall 238 

447. — Cross  Section  of  Exhibition  Hall  through  one  of  the  Domes 238 

448.— Part  Plan  of  Ceiling  of  Exhibition  Hall 238 

449  and  450. — Elevation  and  Horizontal  Section  of  Reinforced  Concrete  Fa9ade  suitable  for  Club  239 

451  and  452. — Detail  of  Reinforced  Concrete  Fa9ade  in  Elevation  and  Vertical  Section  .  .  240 

453. — Small  Hall  with  Mask  Walls  of  Brick  and  Principals  and  Roof  of  Reinforced  Concrete  .  241 
454  to  457. — Longitudinal  Section,  Plan,  Front  Elevation  and  Cross  Section  of  Hall  with  Reinforced 

Concrete  Principals  and  Roof           ...........  242 

458. — T-piece  for  Sand-blast  Apparatus  ...........  244 

459.— A  Type  of  Bush  Hammer  . 244 

460  to  462. — Front  and  End  Elevations  and  Plan  of  "  Granolithic  Plate  "  ....  245 

463.— Sand  Concrete,  1  : 2.  Full  Size 246 

464.— Crushed  Stone  Concrete  (Cement  1,  Yellow  Bank  Sand  2,  and  f-in.  Screen  Stone  3).  Full  Size  246 
465. — Pebble  Concrete  with  Scrubbed  Surface  (Cement  1,  Bar  Sand  2,  and  iVin.  White  Pebbles  3). 

Full  Size 247 

466.— Granite  Grit  Concrete  (Cement  1,  Bar  Sand  2,  and  J-in.  Granite  Grit  3).  Full  Size  .  .  247 

467._Pebble  Concrete  (Cement  1,  Bar  Sand  2,  and  Screened  Yellow  Pebbles  3).  Full  Size  .  .  249 

468.— Sand  Concrete,  1:3 249 

469. — •Californian  State  Normal  School  at  San  Jose  .........  251 

470. — Single-air-space  Mortar  Blocks  (Sand  and  Portland  Cement)  Laid  in  Fireclay,  after  Firing 

and  Quenching  ..............  260 

471. — Double-air-space  Mortar  Blocks,  after  Firing  and  Quenching 260 

472. — Terra-cotta  Three-air-space  Partition  Tiles  Laid  in  Cement  Mortar,  after  Firing  and  Quenching  260 

473. — Various  Building  Stones  Laid  in  Cement  Mortar,  after  Firing  and  Quenching  .  .  .  260 

474. — Concrete  Blocks  Laid  in  Fireclay,  after  Firing  and  Quenching 261 

475. — Granite  Blocks  Laid  in  Cement  Mortar,  after  Firing  and  Quenching  .  .  .  .261 

476. — Graph  Showing  Relative  Water  Absorption  of  Concrete 267 

477. — Section  through  Floor  Slab  supported  by  Two  Beams       .......  273 

478. — Cross  Section  through  Beam  Centering  ..........  274 

479.— Horizontal  Thrust  at  Crown  of  Arch 288 

480. — Three  Forces  Acting  on  Half  of  Arch  ..........  288 

481.— Finding  Line  of  Thrust  in  Arch •  289 

482. — Forces  acting  on  Arch  to  Produce  Equilibrium  ........  289 

483.— Three-hinged  Arch .  .  .290 

484.— Two-hinged  Arch .  .  290 

485. — Foot-bridges  at  Railway  Terminals,  Vera  Cruz           ......          .          •          •          •  290 

486. — Foot-bridge  at  Railway  Terminals,  Vera  Cruz  ....  ...  291 

487. — Foot-bridge  over  Weisseritz,  at  Cotta-Dresden 292 

488. — Moortown  Bridge.  Wimborne,  Dorset 293 


xviii  LIST    OF    ILLUSTRATIONS 

FIO.  PAGE 

489. — Decking  and  Parapets  of  Bridge  in  Cowley  County,  Kansas      ......  293 

490. — Underneath  View  of  Main  Floor  Span  of  Bridge  in  Cowley  County,  Kansas         .          .          .  294 

491. — Elevation  and  Details  of  Bridge  in  Cowley  County,  Kansas       ......  294 

492. — Bridge  over  Merrimac  River,  New  Hampshire            ........  295 

493. — Foot-bridge  at  Mizen  Head 296 

494. — Erecting  the  Eibs  for  Foot-bridge  at  Mizen  Head     ........  296 

495. — Suspended  Centering  for  Bridge        ...........  297 

496. — Bridge  Crossing  the  Bremba,  Italy  .          . 297 

497.— Bridge  at  Kiel  Dock  Works 298 

498. — Oceanside  Bridge  over  San  Luis  Key  River,  California      .......  300 

499.— Scenery  Hill  Viaduct,  Philadelphia  . 301 

500.— Meadow-Street  Bridge,  Pittsburg 302 

501. — Sectional  Plan,  Elevation  and  Details  of  Meadow  Street  Bridge,  Pittsburg          .          .          .  303 

502.— Grafton  Bridge,  Auckland 304 

503. — Bridge  over  River  Werra,  Thuringia         .          .          .          .          .          .          .          .          .          .  306 

504. — Elevation  and  Plan  of  Bridge  over  River  Werra       ........  307 

505. — Details  of  Beams  for  Bridge  over  River  Werra         ........  307 

506. — Plan  and  Elevation  of  Walnut  Lane  Bridge,  Philadelphia 308 

507. — Bridge  near  Teufen,  Switzerland 309 

508. — Retaining  Wall  with  Asphalt  Damp-proof  Course .311 

509. — Timbering  to  Support  Earth,  before  Building  Retaining  Wall  (R.  A.  C.'s  Building)         .          .  312 

510. — Shuttering  and  Reinforcements  for  Retaining  Wall  (R.  A.  C.'s  Building).          .          .          .  312 

511.—  Retaining  Wall  Complete  (R.  A.  C.'s  Building) .313 

512. — Inside  Face  of  Retaining  Wall,  and  Methods  of  Supporting  Mains  (R.  A.  C.'s  Building)       .  314 

513.— Section  Showing  Front  Vaults  (R.  A.  C.'s  Building) 315 

514.— Plan  of  Grillage  Foundation  (R.  A.  C.'s  Building) 315 

515. — Section  through  Top  Layer  Beams  of  Grillage           ........  315 

516. — Section  through  Bottom  Layer  Beams  of  Grillage     ........  315 

517. — Trench  for  Grillages  under  Stanchions  (R.  A.  C.'s  Building)        ......  316 

518. — Asphalt  Damp-proof  Course  under  Base  of  Back  Retaining  Wall  (R.  A.  C.'s  Building)           .  317 

519.— Section  of  Steel  Pile 317 

520.— Top  of  Finished  Steel  Piling 318 

521.— Cast-iron  Cap  to  Head  of  Steel  Pile 318 

522. — Reinforcement  and  Centering  for  Floor  (R.  A.  C.'s  Building) 319 

523. — Elliptical  Reinforced  Concrete  Lintels 320 

524. — Shuttering  to  Curb  of  Skylight  over  Elliptical  Vestibule  (R.  A.  C.'s  Building)   .          .          .321 

525. — Shuttering  and  Reinforcements  for  Sloping  Roof  (R.  A.  C.'s  Building)         ....  322 

526. — Shuttering  and  Reinforcements  for  Sloping  Roof  (R.  A.  C.'s  Building)         ....  323 

527. — Reinforcement  around  Elliptical  Openings  in  Sloping  Roof  (R.  A.  C.'s  Building)         .          .  324 

528.— Shuttering  for  Swimming  Bath 325 

529. — Reinforcements  for  Swimming  Bath  (R.  A.  C.'s  Building) 325 

530. — Reinforcements  for  Swimmng  Bath  (R.  A.  C.'s  Building) 326 

531. — Reinforcing  Sides  of  Bath  and  Concreting  First  Layer  of  Bars  in  the  Bottom  .          .          .  327 

532. — System  of  Handling  Concrete  at  Royal  Automobile  Club's  Building           ....  329 

533. — Columns  and  Arched  Ribs  in  the  Royal  Liver  Building    .......  330 

534. — Arched  Beam  (before  Concreting)  in  Royal  Liver  Building        .          .         .         .          .          .331 

535.—"  Skeleton "  of  Royal  Liver  Building .  331 

536.— Cathedral  at  Manila,  Philippine  Islands 332 

537. — Various  Sections  through  Nave  of  Manila  Cathedral          .......  333 

538. — Longitudinal  Section  (looking  north)  and  Details  of  Manila  Cathedral        ....  334 

539  and  540.— West  and  East  Elevations  of  Manila  Cathedral 334 

541. — Interior  View  of  Manila  Cathedral  (Looking  West)  ........  335 

542. — Interior  View  of  Manila  Cathedral  (Looking  East) 335 

543. — North  Elevation  of  Manila  Cathedral  with  Plan  of  Organ  Room  (A)  and  Section  of  Window 

Jambs  (B) 336 

544. — Details  of  Nave,  Manila  Cathedral 336 

545.— Details  of  Chancel 336 

546. — Longitudinal  Section  through  Poti  Cathedral 337 

547  and  548. — Elevation  and  Horizontal  Section  of  Dome  of  Poti  Cathedral 338 

549. —Ground  Plan  of  Poti  Cathedral        .                   338 


LIST    OF    ILLUSTRATIONS  xix 

FIG.  PAGE 

550  and  551. — Cross  Section  and  Plan  of  Wesleyan  Hall  .......     339 

552. — Key  Plan  showing  Positions  of  Beams  over  Tea  Room,  Wesleyan  Hall     ....     340 

553. — Section  and  Plan  of  Arch  over  Tea  Room,  Wesleyan  Hall        .          .          .          .          .          .     340 

554.— A  Typical  Pillar,  Wesleyan  Hall 340 

555  and  556. — Details  of  Ceiling  and  Floor  over  Tea  Room,  Wesleyan  Hall  ....  341 
557  and  558. — Details  of  Arched  Ceiling  to  Basement  Hall,  Wesleyan  Hall  ....  342 
559. — Cross  Section,  through  Part  of  Arched  Ceiling  to  Basement  Hall,  Wesleyan  Hall  .  .  .  343 

560.— A  Beam  in  the  Floor  of  Conference  Hall,  Wesleyan  Hall 343 

561  to  564. — Sections  through  Floor  of  Conference  Hall,  Wesleyan  Hall     .....     344 

565. — Section  through  and  Details  of  East  Gallery,  Wesleyan  Hall     ......     345 

566. — Free  Church,  Hampstead  Garden  Suburb,  in  Course  of  Construction  ....     346 

567. — South,  West,  and  East  Elevations  of  Free  Church,  Hampstead  Garden  Suburb  .          .     347 

568  and  569. — South  Elevation  and  Ground  Plan  of  St.  Barnabas'  Church,  Jamaica       .          .          .     348 
570. — Longitudinal  Section  through  St.  Barnabas'  Church,  Jamaica    ......     349 

571  and  572. — East  and  West  Elevations  of  St.  Barnabas'  Church,  Jamaica  •  .          .          .     349 

573.— North  Elevation  of  Chancel 349 

574. — Details  of  the  Reinforcement,  St.  Barnabas'  Church,  Jamaica  ......     350 

575. — Sections  through  Nave  and  Chancel,  St.  Barnabas'  Church,  Jamaica  .          .          .    •  351 

576. — Balcony  and  Cantilevers  in  Majestic  Theatre,  Los  Angeles         ...          .          .          .     351 

577. — Cantilevers  at  Majestic  Theatre,  Los  Angeles   .........     352 

578. — Trussed  Girder  at  Majestic  Theatre,  Los  Angeles      ........     352 

579  and  580. — Trussed  Girder  in  Majestic  Theatre,  Los  Angeles          ......     353 

581. — Roof  Plan  and  Details,  National  Gallery  Extension.          .......     354 

582. — Details  of  Intermediate  and  End  Ribs  of  Dome  Roof  to  National  Gallery  Extension     .          .     355 
583  and  584. — Dome  Roof  to  National  Gallery  Extension  .......     355 

585. — Detail  of  Ribs  to  Dome,  National  Gallery  Extension         .          .          .          .          .          .          .     356 

586. — Detail  of  Intermediate  Ribs,  Northern  Gallery  ........     356 

587.— Detail  of  Intermediate  Ribs,  West  Gallery 356 

588.— Detail  of  Pendentive .         .          .356 

589.— Detail  of  End  Ribs,  Northern  Gallery      .          .          . 356 

590.— Detail  of  End  Ribs,  West  Gallery 356 

591. — Bunker  at  Kinlochleven  Aluminium  Works       .........     357 

592. — Section  through  Bunker  at  Kinlochleven  Aluminium  Works      .  .     357 

593.— Detail  of  Beam  of  Bunker  (A,  Fig.  592)  ...  .357 

594  and  595.— Plan  and  Section  of  Foundation  Slab,  Royal  Insurance  Building  .  .     358 

596. — Details  of  Strong  Room  in  Sub-basement,  Royal  Insurance  Building  .     359 

597  and  598. — Vertical  and  Horizontal  Sections  of  Dome  over  Main  Entrance,  Royal  Insurance 

Building .360 

599  and  600.— Walls  with  Post  Buttresses         ...  .361 

601  to  604.— Moulding  Box  for  Fence  Posts      .          .  .362 

605.— Fence  Posts .362 

606. — Reinforcement  for  Fence  Post          ....  •     363 

607. — Iron  Straining  Post •  •     363 

608  to  610.— Reinforcement  and  Shuttering  to  Wall  .  .     364 

611.— Retaining  Wall,  Dilworth  Street,  Pittsburg       .  .364 

612.— Reinforcement  of  Heel  of  Retaining  Wall •  364 

613  to  616.— Photographs,  Elevation,  Plan  and  Sections  of  Spiral  Staircase        .  .     365 

617. — Stairway  in  Mclntyre  Building,  Salt  Lake  City         ...  .367 

618.— Staircases  in  Ritz-Carlton  Hotel,  New  York     . 
619.— 247-ft.  Chimney  at  Northfleet 

620. — 144-ft.  9-in.  Chimney  near  Drury  Lane,  London •          •     369 

^70 
621. — Sewer  Pipe  Reinforcements,  Paris 

622.— Sewer  Construction  at  St.  Louis,  U.S.A. 

623  and  624. — Sewer  Construction  at  Acton,  London 

625.— Cross  Section  of  Culvert  at  Kilton  . 

626. — End  Elevation  of  Culvert  at  Kilton 

627. — Cross  Section  of  Conduit  at  Woolwich     . 

628.— Wharf  at  Dennemont      .... 

629. — Details  of  Wharf  at  Dennemont     . 

630.— Details  of  Jetty  Head  at  Thames  Haven          ....  ...     375 


LIST   OF    ILLUSTRATIONS 

FIO.  PAGE 

631  and  632. — Jetty  Head  at  Thames  Haven,  in  Course  of  Construction    .....  376 

633  and  634. — Piles  and  Bracing  of  Thames  Haven  Jetty  Head         ......  377 

635. — Lifeboat  Slipway,  Ackergill      ............  378 

636  to  639.— Lifeboat  Slipway,  Ackergill 379 

640  and  641. — Columns  and  Arched  Roofing  of  Kloof  Nek  Reservoir          .....  380 

642. — Bridge  forming  part  of  Brooklands  Motor  Track      ........  381 

643.— Stadium  at  Shepherd's  Bush,  London 382 

644. — Platform  Construction,  Stadium 382 

645. — Bradford  Football  Stand  in  Course  of  Construction            .......  383 

646. — Floors,  Columns,  and  Beams,  Bradford  Football  Stand 383 

647. — Details  of  Foundations  and  Columns,  Bradford  Football  Stand          .....  384 

648.— Plans  of  Bradford  Football  Stand 384 

649. — Details  of  Beams  and  Brackets,  Bradford  Football  Stand          ......  385 

650. — Various  Sections  through  Bradford  Football  Stand  ........  385 

651. — Grand  Stand,  St.  Paul,  in  Course  of  Construction    ...          .          .          .          .          •          •  386 

652. — Another  View  of  Grand  Stand,  St.  Paul,  in  Course  of  Construction  .....  387 

653.— East  and  West  Elevations  of  Grand  Stand,  St.  Paul 388 

654.— Half-elevation  (to  street)  of  Grand  Stand,  St.  Paul  .          .          .          .      %    .          .          .388 

655. — Railway  Sleepers    ..............  389 

656.— Details  of  Sleepers 389 

657  and  658.— Two  Sections  through  Silos 390 

659.— Section  through  Silo  Walls 390 

660.— Horizontal  Section  through  Silos 390 

661. — Horizontal  Section  through  Silo  Roof       ..........  390 

662. — Horizontal  Section  through  Silos  (looking  upwards)           .......  390 

663. — Horizontal  Section  through,  showing  Tunnel  Arrangement         ......  390 

664  and  666.— Water  Tower  at  Singen 391 

666.— Vertical  Section  of  Water  Tower 392 

667  and  668.— Three  Horizontal  Half-sections  of  Water  Tower  .                                                           .  392 


Introduction;  What  Reinforced  Concrete  Is 


The  Limitations  of  Concrete.— For  thou- 
sands of  years  concrete  has  been  known 
and  used  as  a  building  material  possessing 
many  valuable  properties — universality  and 
consequent  cheapness ;  ease  of  handling, 
placing,  and  shaping  ;  ability  to  resist  fire, 
water,  and  other  destructive  influences  ;  and 
great  strength  under  compression.  (A 
body,  itself  evenly  and  rigidly  supported,  is 
under  compression  when  a  load  is  placed  on 
it  tending  to  squeeze  it.)  Until  the  twenties 
of  the  nineteenth  century,  concrete  was  made 
by  mixing  together  stones,  gravel,  etc.,  and 
lime,  but  such  was  the  weakness  of  the 
material  in  tension  (that  is,  when  subjected 
to  a  force  that  tended  to  stretch  it  or  bend 
it)  that  its  employment  had  to  be  restricted 
to  such  applications  as  foundations,  rela- 
tively thick  walls,  and  the  like.  On  the 
introduction  of  portland  cement  in  1824, 
the  quality  of  concrete,  particularly  as 
regards  the  tensile  strength,  very  greatly 
improved  ;  and  modern  cement  manufacture 
— a  highly  specialised  industry — has  pro- 
duced a  material  with  an  appreciable  ability 
to  resist  a  stretching  force.  But  good  as 
the  best  cement  is,  it  is  still  not  good  enough 
for  use  in  a  structure  by  itself  except  under 
compression. 

Reinforcement. — Fortunately,  it  early- 
occurred  to  some  ingenious  workers  that  if 
they  could  use  with  it  a  second  substance 
which  would  supply  the  tensional  strength 
lacking  in  the  concrete,  they  would  then  be 
in  possession  of  a  well-nigh  ideal  material 
for  structural  purposes.  Tile,  wood,  bronze, 
iron,  and  steel  have  all  been  tried,  and  the 
preference  goes  to  the  last-named.  Modern 
reinforced  concrete,  then,  is  simply  a  com- 
bination of  a  material  (concrete)  strong  in 
compression  but  weak  in  tension  with  one 
(steel)  that  is  itself  strong  in  compression, 
but  so  much  more  strong  relatively  in 
tension. 

It  may  occur  to  some  readers  to  ask  why 
.  steel  should  be  used  to  bolster  up  a  deficiency 


in  another  material.  Why,  it  might  be 
asked,  is  not  the  steel  used  alone,  and  full 
advantage  taken  of  its  valuable  qualities  ? 
Steel-frame  construction  was  earlier  in  the 
commercial  field  than  reinforced  concrete, 
but  it  has  not  prevented  the  latter  system 
from  forging  ahead  and  making  a  truly 
notable  progress.  There  must  be  a  good 
reason  for  that,  and  it  is  to  be  found  in  a 
number  of  considerations.  Steel  is  relatively 
expensive  ;  the  minimum  quantity  is  used 
in  reinforced  concrete  construction.  A  steel 
structure  involves  the  making  of  thousands 
of  riveted  joints,  and  calls  for  a  large  amount 
of  skilled  work ;  reinforced  concrete  con- 
struction is  monolithic  (literally,  "as  one 
stone  "),  and  there  is  practically  no  jointing. 
Steel  is  corroded  by  atmospheric  action 
unless  immediately  protected,  and  the  pro- 
tection needs  periodic  renewal ;  concrete  is 
scarcely  affected  by  atmosphere,  requires 
no  protection  and  no  maintenance,  and  it 
preserves  the  steel  even  brighter  than  when 
it  was  inserted ;  indeed,  steel  soon  loses  a 
coating  of  rust  when  embedded  in  concrete. 
Most  important  of  all,  an  unprotected  steel 
structure  is  the  worst  possible  for  resisting 
fire,  by  reason  of  the  metal  being  such  a 
good  conductor  of  heat ;  the  steel  rapidly 
expands  as  the  temperature  rises  and  con- 
tracts again  as  the  cold  water  from  the  hose 
reaches  it,  thus  pushing  and  pulling  apart 
the  elements  of  the  structure  and  often 
causing  complete  ruin ;  then  its  softness, 
should  an  extreme  temperature  be  reached, 
robs  it  of  its  strength,  and  the  steel  girders 
become  bent  and  twisted  into  a  mere 
entanglement.  On  the  other  hand,  concrete 
is  a  poor  conductor  of  heat,  and  therefore  well 
protects  the  embedded  steel  and  localises 
any  ill  effects.  Keinforced  concrete  possibly 
brings  the  goal  of  an  absolutely  fireproof 
structure  within  the  attainment  of  our  own 
time. 

Terminology. — Keinforced    concrete,   in 
the  commercial  acceptance  of  the  term,  is 


REINFORCED    CONCRETE 


concrete  in  which  steel  rods  have  been  em- 
bedded to  increase  its  strength.  The  term  is 
somewhat  vague,  but  no  completely  satis- 
factory substitute  for  it  has  been  suggested. 
The  material  was  (and  often  is)  known  on 
the  Continent  as  "  Monier  concrete,"  but 
this  does  not  explain  itself,  and  may  there- 
fore be  dismissed.  "  Ferro-concrete  " — one 
of  the  best  terms  yet  introduced — has  been 
monopolised  as  a  trade  name,  although  it  is 
used  as  a  generic  term  in  technical  literature  ; 
even  that  designation,  though,  is  not  strictly 
correct,  since  "  ferro  "  of  course,  is  formed 
from  "  ferrum  "  or  "  fer  "  (respectively  Latin 
and  French  for  iron) ;  but  iron,  as  a  struc- 
tural material,  has  long  given  place  to 
steel,  made  by  "  alloying  "  iron  with  carbon. 
The  term  "  reinforced  concrete  "  does  not, 
unfortunately,  suggest  the  material  employed 
with  the  concrete  ;  for  example,  at  an  early 
date  concrete  was  reinforced  with  wood ; 
indeed,  wood-reinforced  concrete,  known  as 
"  ligno-concrete,"  is  now  attracting  some 
attention,  but  the  term  under  discussion 
does  not  differentiate  between  "  ferro- 
concrete," "  ligno-concrete,"  or  "sesso-con- 
crete  "  (bronze-concrete),  the  earliest  type  of 
reinforced  concrete  known.  The  term  is 
open  to  another  objection ;  in  a  sense,  the 
steel  is  itself  just  as  much  "  reinforced " 
as  is  the  concrete,  and  therefore  "  rein- 
forced steel "  might  seem  as  logical  a  de- 
signation. The  term  "  concrete-steel  "  has 
its  advocates,  amongst  whom  we  beg  to 
take  our  place ;  but  it  has  not  caught 
the  imagination  to  the  extent  that  some  of 
the  other  terms  have.  Much  the  same 
can  be  said  of  "  armoured  concrete."  It 
appears  that  the  one  term — other  than 
trade  names — likely  to  become  universal  is 
"  reinforced  concrete,"  and  for  that  reason 
it  has  been  adopted  as  the  title  of  this  book, 
the  more  precise  "  concrete-steel "  finding 
a  place  in  the  sub-title. 

A  Compound  Material. — Concrete-steel 
may  be  regarded  as  a  compound  material, 
and  not  as  two  distinct  materials.  The  steel 
is  wholly  embedded  in  the  concrete,  and  the 
adhesion  between  the  two,  after  the  maturing 
of  the  concrete,  is  such  that  a  force  sufficient 
to  pull  the  bars  from  the  concrete  would  re- 
quire to  be  500  Ib.  to  600  Ib.  per  square  inch 
of  surface  contact,  although  it  is  regarded 
as  safe  to  allow  only  100  Ib.  per  square  inch. 
Further,  the  two  materials  may  be  con- 
sidered as  expanding  and  contracting  at 
the  same  rates.  The  coefficient  of  expansion 


of  concrete  is  -000006  (per  1°  F.  change  of 
temperature),  and  that  of  steel  averages 
•0000066 ;  the  difference  is  therefore  only 
1  in  about  a  million  and  three-quarters. 
In  spite  of  all  this,  there  exists  a  division 
in  the  ranks  of  the  concrete-steel  engineers 
and  specialists.  On  the  one  hand  are  those 
who  believe  that  the  natural  adhesion  of  the 
concrete  to  the  steel  is  sufficient  to  answer 
all  ordinary  requirements.  On  the  other  are 
those  who  say  that  experience  teaches  that 
this  adhesion  ought  to  be  assisted  by  a 
mechanical  bond,  and  they  therefore  "  de- 
form "  the  bar  to  remove  the  least  likelihood 
of  its  slipping  when  subjected  to  severe  ten- 
sion, this  being  a  time  at  which  the  sectional 
area  of  the  bar  is  liable  to  be  reduced,  thus 
lessening  or  destroying  the  adhesion.  De- 
formed bars  are  also  used  to  allow  of  the 
employment  of  a  steel  having  a  greater 
tensile  strength  than  mild  steel.  The  chief 
Continental  systems  use  plain  bars,  while 
the  chief  American  systems  use  bars  of 
special  shape.  This  subject  suggests  other 
considerations,  which  would,  however,  lead 
us  too  far  into  the  theory  of  the  matter, 
and  which  will  therefore  be  relegated  to  a 
later  chapter. 

Comparisons. — Stated  in  approximate 
terms,  the  compressive  strengths  of  concrete 
and  steel  are  as  1  :  28  ;  the  tensile  strengths 
as  1  :  280 ;  and  the  weights,  bulk  for  bulk, 
1:4.  To  these  ratios  may  be  added  those 
of  cost,  bulk  for  bulk ;  let  us  assume  the 
approximate  costs  of  good  concrete  (mate- 
rials only)  and  steel  to  be  1  :  80,  although 
this  of  necessity  will  vary.  Working  on 
these  data,  some  interesting  comparisons 
may  be  made.  Figs.  1  to  3  show  sections 
of  circular  columns,  all  assumed  to  be  of 
the  same  height  and  capable  theoretically  of 
supporting  the  same  weight.  The  concrete 
column  must  be  28  times  the  area  in  cross- 
section  of  that  of  the  steel  column,  and 
nearly  1|  times  that  of  the  concrete-steel 
one ;  roughly,  the  relative  areas  would  be 
about :  steel,  4  ;  plain  concrete,  112  ;  rein- 
forced concrete,  80.  This  comparison  does 
not  adequately  show  the  true  advantage  of 
reinforced  concrete,  since  the  reinforcement 
in  a  tall  concrete  column  is  absolutely  neces- 
sary, the  rods  and  hooping  preventing  the 
concrete  from  bursting  under  a  heavy  load. 
Again,  in  the  case  of  eccentric  loading,  one 
side  of  the  column  may  be  in  tension,  and 
steel  is  then  required  to  take  this  stress. 
The  relative  sizes  will  depend  upon  the 


INTRODUCTION 


percentage  of  reinforcement,  and  the  fore- 
going is  based  upon  the  minimum  usually 
employed.  In  the  case  given,  the  area  of 
steel  in  the  steel  column  is  twice  that  in  the 
reinforced  column,  and  it  is  this  material 
that  is  the  more  expensive  of  the  two. 
Again,  the  weight  of  the  concrete  column 


the  relative  costs  (material  only)  working 
out  as  (roughly)  6-2,  1-7,  and  1.  This 
economy  of  steel  is  possible  because  all  the 
steel  in  the  reinforced  concrete  beam  is 
placed  as  far  as  possible  from  the  "  neutral 
axis,"  and  can  therefore  be  stressed  and 
utilised  to  its  full  value ;  whereas  in  the 


AEEA-200 

Fig.  2 


AEEA-  5600 

Fig.  1 


S>[EEL-  100  £<3.  INO. 
TOJAL  M?E.A-  4000  SQ.  IH5 
Fig.  3 


Figs.   1  to  3. — Columns  of  Plain  Concrete,   Steel,  and  Reinforced  Concrete,  respectively  : 

Comparative  Diagrams 


is  7  times  that  of  the  steel  one,  and  about 
1*3  times  that  of  the  concrete-steel ;  but 
the  costs  (for  materials  alone)  will  be  approxi- 
mately in  the  proportions  of  7,  20,  and 
(nearly)  15. 

Now  consider  three  beams,  respectively  of 
concrete,  steel,  and  reinforced  concrete,  of 
the  same  depth  and  of  the  same  value  of 
resistance — that  is,  capable  of  supporting 
the  same  load.  Figs.  4,  5,  and  6  are  cross 
sections  of  these  beams,  d  indicating  depth 


steel  beam,  much  of  the  metal  is  adjacent 
to  the  neutral  axis,  and  part  of  its  value  is 
lost.  (When  a  beam  is  loaded  reasonably, 
it  tends  to  bend,  and  its  length  is  altered, 
but  there  is  a  layer  of  fibres  or  a  plane,  the 
length  of  which  remains  unaltered ;  in  a 
cross-section  this  "  neutral  surface  "  is,  of 
course,  a  line,  which  is  known  as  the  "  neutral 
axis.") 

It   will  be  understood  by  the  practical 
reader  that  the  diagrams  to  this  chapter  are 


A 

X 

i 

PLAIN   CONCRETE: 


Fig.  4 


ARtA=  3-3625 1  AietA  or ajttL = '67^ i 

Fig.  6 


(Hi. 
Fig.  5 


Figs.  4  to  6. — Beams  of  Plain  Concrete,  Steel,  and  Reinforced  Concrete,  respectively  : 

Comparative  Diagrams 


(the  same  in  all  three  cases)  and  6  breadth. 
Expressed  in  inches,  the  concrete  beam  has 
a  breadth  of  76  in.,  the  steel  beam  -285  in., 
and  the  reinforced  concrete  beam  8  in.  Their 
relative  weights  are  (roughly)  70,  1,  and  7£. 
this  might  seem  to  show  an  advantage  for 
the  steel  beam,  but  it  will  be  noted  that  the 
steel  beam  contains  nearly  five  times  as 
much  steel  as  the  reinforced  concrete  beam, 


purely  comparative,  as  steel  is  not  commonly 
used  in  the  form  shown.  If  for  the  rect- 
angular steel  beam  were  substituted  a  rolled 
steel  joist  having  an  equivalent  moment  of 
resistance,  its  area  would  exceed  3£  sq.  in., 
that  is,  more  than  five  times  that  of  the 
steel  in  the  reinforced  concrete  beam. 

These  comparisons  show  at  a  glance  that 
it  is  cheaper  to  use  concrete  than  steel  to 


REINFORCED    CONCRETE 


withstand  compression  ;  but  that  to  with- 
stand tensional  stress  concrete  theoretically 
would  cost  two  and  a  half  times  as  much  as 
the  steel,  but,  practically,  would  be  im- 
possible. Keinforced  concrete  effects  the 
compromise  between  the  two ;  where  it 


costs  more  than  either  steel  or  concrete 
alone,  it  offers  advantages  not  possessed 
by  the  material  with  which  it  is  compared. 
The  parallel  columns  below  put  the  case  for 
the  employment  of  reinforced  concrete  at 
a  glance. 


CONCRETE 


STEEL  (MILD) 


REINFORCED  CONCRETE 


Weak  in  tension 
Strong  in  compression 
Cannot  resist  a  strong  shearing 

force 
A  relatively  cheap  material 

Very  heavy,  strength  for  strength 

Fire-resisting 

Not  attacked  by  weather  and 
atmosphere ;  low  mainten- 
ance cost 

Easy  to  place  and  shape 


Very  strong  in  tension 
Very  strong  in  compression 
Can    resist     a    fairly     strong 

shearing  force 
A  relatively  expensive  material 

Very  light,  strength  for  strength 

Destroyed  by  fire 
Rapidly  oxidised  ;    high  main- 
tenance cost 

Difficult  to  place  and  shape 


Strong  in  tension 

Strong  in  compression 

Can  be  made  to  resist  a  strong 
shearing  force 

Price  competitive  with  that  of 
any  other  system 

Lighter  than  plain  concrete, 
strength  for  strength 

Fire-resisting 

Not  attacked  by  weather  and 
atmosphere ;  low  mainten- 
ance cost 

Easy  to  place  and  shape 


Historical   Notes 


THE  history  of  a  practical  science  is  seldom 
complete  and  proper  without  an  early  refer- 
ence to  the  ancient  Eomans.  The  history 
of  reinforced  concrete  is  not  one  of  the 
exceptions.  Readers  will  be  familiar  with  the 
oft-repeated  statement  that  the  Romans 
commonly  used  this  system  in  the  construc- 
tion of  their  public  buildings  ;  but  while  this 
is  an  exaggeration  of  the  truth,  there  are 
certain  unassailable  facts  that  stand  out 
prominently.  The  Romans  made  good  con- 
crete which  can  be  seen  to-day  ;  they  antici- 
pated ligno-concrete  (wood-reinforced  con- 
crete) ;  they  combined  tiles  and  concrete  ; 
and  they  roofed  the  Frigidarium  of  the  Baths 
of  Caracalla  with  a  coarse  concrete,  made 
with  lime  hydra ulicised  with  trass  or  volcanic 
scoria,  and  reinforced  with  bronze  and  iron 
rods.  It  is  fairly  certain,  however,  that 
there  was  no  general  knowledge  at  the  time 
of  the  principle  of  reinforcement  as  it  is 
understood  to-day.  There  is  no  reinforce- 
ment, be  it  noted,  in  the  Pont  du  Gard,  a 
bridge  in  the  south  of  France  erected  about 
56  B.C.,  and  still  in  existence — a  worthy 
memorial  to  honesty  of  construction  ;  in  this 
bridge  the  coarse  material  (stones)  and 
cementing  material  (lime)  were  not  mixed  to- 
gether,  but  were  apparently  placed  in  alter- 
nate layers.  It  is  difficult  to  believe  that 
for  eighteen  centuries  the  idea  applied  by 
the  old  Roman  bath  builder  could  have  lain 
dormant ;  but  while  we  know  that  concrete 
of  a  sort  was  commonly  used,  there  is  no 
evidence  of  the  principle  of  reinforcing  it,  and 
so  extending  its  usefulness,  having  been 
commonly  applied  at  any  time  during  that 
long  period,  unless  exception  is  made  in  the 
case  of  the  dome  of  St.  Paul's  Cathedral 
(1675-1710),  in  which,  as  is  well  known,  Sir 
Christopher  Wren  caused  chains  to  be 
embedded  in  concrete  to  help  in  resisting  the 
lateral  thrust.  Certain  of  these  chains  were 
examined  some  years  ago,  and  were  found  not 
to  have  corroded,  although  the  concrete  had 
been  continually  in  a  damp  state. 

Early  in  the  Nineteenth  Century.— 
It  is  not  until  1830  that  we  find  any  definite 
mention  of  the  idea  of  reinforced  concrete, 
in  which  year  was  published  J.  C.  London's 


"  Encyclopaedia  of  Cottage,  Farm,  and 
Village  Architecture,"  containing  the  sug- 
gestion that  roofs  might  be  constructed  of 
cement  in  which  were  embedded  iron  tie- 
rods  in  the  form  of  a  lattice  work,  the  whole 
being  cased  with  flat  tiles.  Six  years  pre- 
viously (in  1824)  Joseph  Aspdin  patented 
his  method  of  making  (portland)  cement, 
and  it  had  already  (in  1828)  been  used 
extensively  in  the  construction  of  the  Thames 
Tunnel ;  but  for  many  years  it  had  to  fight 
opposition  and  was  not  in  general  use.  For 
example,  in  the  year  1840,  when,  as  the 
result  of  a  strike  of  carpenters,  some  fire- 
proof floors  were  constructed  in  Paris,  they 
failed  to  become  popular  because  a  local 
material,  gypsum  (calcined  to  form  what  is 
now  known  as  "  plaster-of- Paris  "),  was  used 
as  the  cementing  material  in  the  concrete, 
the  result  being  that  the  embedded  iron  soon 
rusted.  These  floors  were  described  in  a 
paper  read  before  the  Royal  Institute  of 
British  Architects  in  1849  by  G.  R.  Burnell, 
and  more  fully  described  five  years  later 
before  that  body  by  H.  H.  Burnell.  Two 
systems  were  known  ;  in  one — the  Vaux — 
round  rods,  close  together,  were  hooked  at 
each  end  on  to  a  flat  wrought-iron  bar  lying 
on  its  edge  ;  in  the  other — the  Thuasne — 
iron  joists  were  employed,  stirrups  hanging 
from  these  and  containing  holes  through 
which  the  round  reinforcing  rods  passed. 
In  each  case  the  ironwork  was  embedded 
in  plaster  concrete. 

About  the  same  time  (July  9,  1840)  a 
reinforced  ceiling  slab  was  patented  by  a 
Frenchman,  Louis  Leconte ;  he  proposed 
"  the  use  of  trusses  of  iron  plates  for  floors, 
from  which  iron  rods  were  suspended  to  carry 
a  wire  meshwork  for  sustaining  plaster." 
There  is  evidence  of  an  indefinite  nature  that 
shortly  before  this  period  English  architects 
had  constructed  fireproof  floors  of  concrete 
or  brickwork  in  which  flat  iron  bars  were 
embedded. 

A  Concrete  Boat. — A  fertile  period  of 
invention,  as  relating  to  the  subject  under 
consideration,  was  the  fifties  of  the  nine- 
teenth century.  Lambot,  a  French  con- 
tractor, had  proposed  to  build  the  hulls  of 


REINFORCED    CONCRETE 


boats  with  concrete,  and  at  the  first  Paris 
International  Exhibition  (1855)  he  actually 
showed  a  flat-shaped  boat,  with  sides  2  in. 
thick,  made  of  hydraulic  lime  concrete 
reinforced  with  a  skeleton  of  iron  rods  ;  and 
in  the  same  year  he  patented  his  invention 
in  Great  Britain.  At  the  present  time, 
boats  are  often  built  of  reinforced  concrete. 
Wilkinson. — In  spite  of  the  systems  that 
had  been  suggested,  and  of  those  that  had 
already  been  practically  tried,  the  chief 
credit  for  the  invention  of  reinforced  con- 
crete is  commonly  ascribed  to  William 
Boutland  Wilkinson,  who,  on  October  27, 
1854,  patented  a  method  of  constructing  a 
fire-resisting  floor  of  concrete  slabs  rein- 
forced with  a  network  of  flat  iron  rods 
placed  on  edge  or  with  secondhand  wire 
ropes.  Wilkinson  was  a  Newcastle-on-Tyne 
plasterer,  and  as  by  1852  there  was  estab- 
lished at  Gateshead-on-Tyne,  by  William 
Aspdin,  a  cement  factory,  it  may  be  fairly 
assumed  that  Wilkinson  was  thoroughly 
familiar  with  the  use  of  cement  concrete. 
We  cannot  do  better  than  quote  from  the 
admirable  "  History  "  printed  in  the  "  Lock 
Woven  Mesh  Handbook."  Wilkinson  it  was 
"  who  first  suggested,  in  his  1854  patent, 
the  use  of  a  layer  of  sand  kept  wet  upon  the 
surface  of  a  freshly  made  concrete  floor,  for 
the  purpose  of  allowing  the  concrete  to  gain 
the  maximum  hardness.  His  patent  also 
included  the  construction  of  hollow  partition 
blocks  very  similar  in  detail  to  the  inter- 
locking plaster  partitions  of  to-day.  The 
chief  object,  however,  of  the  patent  was  the 
construction  of  fire-proof  floors ;  these 
Wilkinson  proposed  constructing  both  in  arch 
form  and  flat,  and  he  suggested  reinforcing 
them  with  either  flat  iron  bars  placed  on 
edge,  or  with  secondhand  wire  rope.  Wil- 
kinson states  that  the  reinforcement  was  to 
be  placed  in  the  concrete  to  take  the  tension, 
and  his  drawings  clearly  show  that  he 
thoroughly  understood  the  construction  of 
such  floors,  for  the  reinforcement  is  placed 
in  positions  in  the  arch  construction  where 
it  could  give  maximum  service,  while  in 
floor  slabs  the  rods  or  wire  ropes  were  bent 
down  at  the  centre  of  the  span  where  the 
maximum  bending  moment  occurs,  but  in 
continuous  spans,  or  where  built-in,  were 
carried  up  to  the  top  over  the  supports,  so 
as  to  resist  the  reverse  bending  moment. 
The  ends  of  the  rope  were  also  directed  to 
be  frayed  out  so  as  to  bond  in  with  the 
concrete  at  the  ends  of  the  span  ;  the  advis- 


ability of  this  was  evidently  fully  appre- 
ciated by  Wilkinson,  namely,  to  give  anchor* 
age  or  mechanical  bond  and  serve  the  same 
purpose  as  the  fishtail  or  bend  at  the  end 
of  a  rod  in  other  systems  of  a  later  date. 
He  also  described  and  illustrated  independent 
beams  of  reinforced  concrete.  His  patent 
showed  a  grasp  of  most  of  the  principles  of 
modern  reinforced  concrete  construction,  and 
it  is  obvious  that  it  was  drawn  up  from 
thorough  practical  experience  and  perhaps 
some  theoretical  grasp  of  the  subject." 

In  course  of  time,  Wilkinson  erected  a 
number  of  reinforced  concrete  buildings  in 
Newcastle  and  elsewhere  in  the  North  of 
England,  of  which,  however,  no  records  are 
now  known  to  exist. 

Francois  Coignet. — Less  of  the  builder 
and  more  of  the  engineer  was  the  French 
contractor,  Francois  Coignet,  who  in  1855 
patented,  both  in  France  and  England,  a 
system  of  forming  floors  by  laying  iron 
"  planks,"  or  rods,  crossing  each  other,  from 
wall  to  wall  of  a  building  ;  iron  beams  could 
be  placed  to  support  the  "  planks  "  or  rods. 
A  false  flooring  is  supported  underneath  the 
reinforcement  and  the  concrete  applied. 
Coignet  had  at  an  earlier  date  invented 
"  Beton  Coignet "  (concrete  for  which  the 
hydraulic  lime  and  aggregates  were  mechan- 
ically mixed  in  certain  proportions),  and  the 
advantage  of  reinforcement  possibly  occurred 
to  him  of  itself,  or  he  may  have  remembered 
the  reinforced  plaster-concrete  experimented 
with  some  years  before.  In  the  years  follow- 
ing the  grant  of  his  patent  he  carried  out  a 
variety  of  works,  including  a  lighthouse  at 
Port  Said  (Egypt),  retaining  walls  in  Paris, 
and  thirty-three  miles  of  aqueduct  for  the 
Paris  water  supply.  He  undoubtedly  com- 
monly applied  his  system  of  reinforcement 
in  the  construction  of  these  works,  but  very 
little  is  known  as  to  the  details. 

Dennett,  Allen,  Ransome,  and  Scott. — 
In  1857  (patent  dated  March  9),  a  Notting- 
ham builder,  C.  C.  Dennett,  took  advantage 
of  the  introduction  of  rolled  iron  joists  and 
constructed  a  floor  with  reinforced  concrete 
arches  resting  on  _L-beams.  The  arch  rein- 
forcement was  of  wood  or  iron.  This  floor 
was  the  prototype  of  the  modern  fireproof 
floor  of  steel  joists  and  concrete. 

The  'sixties  also  saw  a  number  of  not- 
able introductions.  In  1862  (January  30) 
Matthew  Allen,  a  London  builder,  patented 
a  system  of  building  staircases,  floors,  etc., 
with  which  he  afterwards  did  much  business  ; 


HISTORICAL    NOTES 


iron  bars  3  in.  by  J  in.,  on  edge,  were 
embedded  in  concrete  (cement  1,  cinders, 
etc.,  3)  2  ft.  apart  near  the  under-side  of  the 
slab,  the  bars  crossing  to  form  a  network. 
The  method  was  sufficiently  practicable  to 
be  adopted  in  the  construction  of  the 
Columbia  Market,  Shoreditch.  Frederick 
Kansome,  in  1865,  suggested  the  construc- 
tion of  girders,  etc.,  by  moulding  a  kind  of 
cement  around  a  hoop-iron  skeleton. 

The  year  1867  saw  two  important  patents. 
One,  by  an  Englishman,  H.  Y.  D.  Scott, 
related  to  a  floor  of  concrete  with  inter- 
laced rods  and  hoop -iron  or  wire,  supported, 
not  on  joists,  but  on  wrought-iron  tie-rods 
embedded  in  the  concrete,  and  is  of  interest 
because  the  specification  alludes  to  the  use 
of  "  the  tie-rods  and  hoop -iron  taking  the 
tensile  strain,  and  the  concrete  the  com- 
pressible," conclusive  evidence  that  the 
basic  principle  of  reinforced  concrete  was  at 
that  time  understood. 

Joseph  Monier. — The  other,  a  French 
patent  dated  1867,  by  a  Frenchman,  Joseph 
Monier,  was  for  the  construction  of  plant  tubs 
and  the  like  by  means  of  concrete  reinforced 
with  a  meshwork  of  rods  or  wires.  Monier 
started  as  a  gardener,  and  developed  into  a 
manufacturer  of  gardening  tools  and  appli- 
ances. He  thought  of  substituting  for  the 
ordinary  plant  tub  a  vessel  of  concrete,  but, 
finding  this  was  brittle,  he  hit  upon  the  idea 
of  making  a  skeleton  of  iron  network  or 
trellis  and  then  enveloping  it  with  mortar 
or  concrete.  The  second  Paris  International 
Exhibition  (1867)  contained  examples  of 
construction  in  the  Monier  and  the  Coignet 
styles.  Monier  followed  up  his  1867  patent 
with  further  inventions,  patented  in  France 
in  1873,  and,  after  the  Antwerp  Exhibition 
of  1879,  at  which  he  exhibited,  he  sold  his 
inventions  to  G.  A.  Wayss  (of  the  firm  of 
Wayss  and  Co.,  Germany),  who  was  soon 
instrumental  in  introducing  the  new  system 
of  building  into  a  number  of  European 
countries.  So  closely  was  reinforced  con- 
crete identified  with  Monier  on  the  Continent 
outside  France  that  it  was  commonly  known 
as  the  "  Monier  system,"  a  term  replaced  in 
course  of  time,  and  as  other  systems  appeared, 
by  the  German -French  "  eisenbetonbau " 
and  "  betoneisenbau  "  (eisen  [G.],  iron  ; 
beton  [F.],  concrete  ;  and  bau  [G.],  build- 
ing or  construction — that  is,  "  ferro-concrete 
construction  "). 

A  patent  granted  to  T.  Lythgoe  and  H. 
Thornton,  in  February,  1868,  is  of  some 


interest.  The  inventors  illustrate  a  floor 
consisting  of  _L-shaped  iron  bars  suitably 
spaced  and  having  a  concrete  filling  between 
them  ;  a  hoop-iron  reinforcement  laces  the 
bars  together,  and  passes  alternately  over 
and  under  them  ;  the  bars  and  the  reinforce- 
ment are  wholly  embedded  in  the  concrete. 

It  cannot  help  striking  the  student  that 
the  principle  and  almost  the  details  of 
reinforcing  were  re-invented  quite  a  number 
of  times  in  the  middle  of  the  nineteenth 
century.  Under  the  present  laws — British, 
at  any  rate — it  is  doubtful  whether  any  sys- 
tem already  mentioned  as  being  invented  sub- 
sequent to  1840 — when,  as  has  been  shown, 
reinforced  floors  were  constructed  in  Paris — 
could  have  been  granted  a  valid  patent. 
The  Monier  floor,  introduced  in  1873,  is 
much  the  same  as  Allen's,  patented  in  1862  ; 
and  Allen's  was  almost  certainly  anticipated 
by  Wilkinson's,  patented  in  1854. 

Phillip  Brannon. — Brannon's  provisional 
patent  in  1870,  completed  the  next  year 
(October  12,  1871),  shows  that  the  inventor 
had  grasped  the  principle  of  reinforcing  con- 
crete. The  specification  of  1871  shows  a 
concrete  floor  reinforced  with  an  iron  mesh- 
work,  as  well  as  the  application  of  reinforced 
concrete  to  the  construction  of  sea  defences. 
Brannon  was  the  first  to  suggest  a  reinforced 
concrete  pile,  the  longitudinal  reinforce- 
ments being  of  angle-irons  united  by  bars 
riveted  latticewise  across  them,  the  whole 
being  wound  spirally  with  wire.  Brannon's 
patents  were  worked  by  a  company  formed 
by  him  and  known  as  "  The  Monolithic 
Fireproof  and  Sanitary  Construction  Works, 
Ltd.,"  which  in  the  ensuing  years  erected 
several  reinforced  concrete  buildings,  some 
at  Walton-on-the-Naze,  Essex,  and  two 
(houses)  in  Islington. 

Thaddeus  Hyatt. — Between  the  years 
1870  and  1877  much  interest  in  the  "  new  " 
system  of  construction  was  exhibited  by 
Thaddeus  Hyatt,  an  American  who  had 
become  known  from  his  invention  of  glass 
pavement  lights.  It  is  doubtful  whether  the 
published  accounts  of  Monier's  and  Coignet's 
work  in  Europe  had  attracted  his  atten- 
tion. Between  1870  and  1877  he  had  a  series 
of  beams  tested  by  Kirkaldy,  and  in  these 
beams  both  single  and  double  reinforcements 
were  tried.  He  indulged  in  an  interesting 
series  of  experiments,  and  these  led  him  to 
believe  in  the  necessity  of  anchoring  the 
ends  of  the  reinforcing  rods,  for  which  'pur- 
pose he  fitted  them  with  nuts  and  washers 


REINFORCED    CONCRETE 


to  prevent  their  pulling  through  the  con- 
crete. Between  the  years  1873  and  1881 
he  obtained  between  thirty  and  forty  differ- 
ent patents,  most  relating  to  reinforced  con- 
crete ;  and  although  he  did  much  valuable 
work  in  assisting  *  engineers  to  a  proper 
understanding  of  the  theory  of  the  rein- 
forced concrete  beam,  it  does  not  appear 
that  commercially  Hyatt  made  any  success 
with  concrete  construction.  Hyatt's  book 
showed  that  he  regarded  a  reinforced  con- 
crete beam  as  corresponding  to  a  rolled  steel 
joist,  the  steel  rods  being  considered  as 
equivalent  to  the  bottom  flange  and  the 
concrete  as  the  top  flange,  the  neutral  axis 
being  assumed  to  lie  half-way  up  the  beam. 
Later,  a  number  of  engineers  made  a  similar 
mistake. 

From  the  'Seventies  to  the  'Nineties.— 
About  the  year  1870,  a  number  of  reinforced 
concrete  sewer  pipes  were  laid  in  Germany, 
and  these  are  still  in  service.  In  1877  an 
experiment  was  tried  at  Croydon  by  W.  H. 
Lascelles,  who  applied  a  system  based  on 
patents  obtained  earlier,  and  built  a  number 
of  cottages  having  a  timber  framework,  the 
slabs  being  of  concrete  reinforced  with 
diagonal  rods. 

We  now  arrive  at  a  period  in  which  interest 
was  being  taken  in  reinforced  concrete  con- 
struction by  engineers  in  all  the  progressive 
countries  of  Europe.  Angelo  Lanzoni,  of 
the  firm  of  Lanzoni,  Galli  and  Co.,  made 
some  applications  of  the  system  in  Italy  in 
1878,  the  first  Italian  patent  being  taken  out 
by  him  five  years  later,  anticipating  Monier's 
Italian  patent  by  four  months.  In  Switzer- 
land, Monier's  system  was  applied  to  the 
erection  of  small  vaultings  as  far  back  as 
1880.  In  the  same  year,  Rudolph  Schuster 
came  to  terms  with  Monier,  and  in  the  suc- 
ceeding years  carried  out  a  series  of  works 
in  Austria,  these  including  large  numbers 
of  reservoirs  and  vats,  vaulted  flooring,  fire- 
resisting  doors,  etc.  Deep  interest  had  been 
aroused  in  Great  Britain,  but  the  Monier 
system,  for  which  a  British  patent  was  taken 
out,  dated  July  7,  1883,  failed  to  become 
successful  there.  The  activity  was  not  con- 
fined to  Europe,  for  in  the  United  States — 
where,  in  1875,  W.  E.  Wood  had  built  a 
reinforced  concrete  house — E.  L.  Ransome 
was  building  reinforced  concrete  warehouses 
about  1884,  following  later  with  a  factory 
building,  the  Californian  Academy  of  Science 
(architect,  G.  W.  Percy),  about  1888,  and  the 
Museum  Building  of  Leland  Stanford,  Junior, 


University  (architect,  G.  W.  Percy),  in  1892, 
the  last-named  building  containing  spans  of 
45  ft.  and  being  reinforced  throughout.  It 
is  stated  that  the  Academy  building  with- 
stood remarkably  well  the  San  Francisco 
earthquake  of  1906.  In  1884  Ernest  L. 
Ransome  patented  in  the  United  States  a 
twisted  square  bar  reinforcement,  and  in  the 
preceding  year  John  F.  Golding  obtained 
an  American  patent  for  "  slashed  metallic 
screening,"  otherwise  expanded  metal,  which 
was  employed  as  a  lathing  for  plaster, 
its  use  as  a  reinforcement  for  concrete 
dating  from  about  1890.  Benjamin  Scarles 
took  out  American  and  British  patents  as 
far  back  as  1884  for  the  use  of  wire  cloth  as 
lathing,  but  even  he  had  been  anticipated. 

In  the  year  1885  W.  H.  Lindsay  patented 
a  system  of  reinforcement  by  which  steel 
rods  were  passed  through  holes  made  in  the 
webs  of  I- joists,  one  at  the  top  and  one  at 
the  bottom  ;  the  vertical  pairs  crossed,  and 
at  the  point  of  intersection  they  were  made 
to  form  a  loop  through  which  was  threaded 
a  rod  parallel  with  the  iron  joists.  About 
the  same  time,  too,  a  practical  experiment 
was  tried  in  Lincoln's  Inn  Fields,  London, 
a  block  of  offices  with  plain  concrete  walls 
and  reinforced  concrete  floors  being  erected 
to  the  designs  of  William  Simmons. 

From  this  point  onward,  we  must  omit  the 
names  of  many  patentees  and  inventors, 
as  during  the  next  twenty  years  or  so  there 
were  hundreds  of  inventions  relating  to 
reinforced  concrete,  and  scores  of  dif- 
ferent systems  were  exploited  in  Europe 
and  the  United  States. 

In  Hungary  reinforced  concrete  was  first 
applied  in  the  erection  of  stables  at  the 
Artillery  Barracks,  Kassa,  in  1886,  by 
Robert  Wiinsch,  of  Budapest.  To  take  up 
the  thrust  of  the  vault,  the  separate  metallic 
ribs  were  anchored,  and  the  resulting  system 
•was  at  a  later  date  widely  applied  in  Austria 
and  Hungary.  Into  Hungary  the  Monier 
system  was  introduced  in  1887  by  Wayss, 
being  first  adopted  for  a  series  of  barrel- 
arched  bridges.  Merely  mentioning  by  the 
way  that  on  March  27,  1890,  J.  Mayoh 
patented  in  Great  Britain  the  use  of  corru- 
gated plates  on  edge  for  strengthening  con- 
crete floors  ;  that  on  July  7,  1891,  C.  A. 
Day  patented  in  Great  Britain  the  use  of 
wire  lattice  suspended  over  steel  joists  for 
reinforcing  concrete  slabs  ;  and  that  in  the 
same  year  Franz  P.  Meyenberg  patented  in 
the  United  States  a  floor  constructed  of 


HISTORICAL    NOTES 


hollow  terra-cotta  tubes  supported  on  steel 
rods  embedded  in  concrete  above  and  below, 
there  being  loose  stirrups  hooked  round  the 
bottom  rods,  we  may  pass  to  the  period 
in  which,  in  spite  of  all  that  had  been  done 
before  it,  reinforced  concrete  first  compelled 
the  serious  attention  of  all  progressive 
engineers. 

Edmond  Coignet  and  Frangois  Henne- 
bique. — The  two  great  names  are  those 
of  Edmond  Coignet  (son  of  Francois  Coignet) 
and  Francois  Hennebique,  the  former  of 
whom,  by  applying  the  known  principles 
of  mechanics,  evolved  a  system  of  calcula- 
tion that  has  proved  remarkably  truthful, 
and  the  latter  of  whom,  basing  his  methods 
of  calculation  upon  results  obtained  in  prac- 
tice, has  also  made  extremely  important 
contributions  to  the  technical  consideration 
of  the  subject.  Coignet  as  the  scientific 
investigator,  and  Hennebique  as  com- 
mercial organiser,  are  properly  regarded  as 
"  the  pioneers  of  the  modern  evolution  in 
the  art  of  building."  The  story  has  often 
been  told  of  the  opposition  which  Coignet 
had  to  fight  in  getting  the  masonry  of  the 
proposed  new  system  of  main  drainage  in 
Paris  in  1892  replaced  by  reinforced  con- 
crete. He  promised  a  large  saving  of  money 
and  of  time  required  for  construction,  and 
his  system,  which  was  finally  adopted,  was 
carried  out  with  complete  success.  Henne- 
bique, having  organised  a  technical  staff  and 
licensed  a  large  number  of  the  most  influ- 
ential contractors  to  work  his  system,  was 
able  to  secure  between  the  years  1892  and 
1899  work  to  the  total  value  of  two  million 
sterling,  representing  three  thousand  con- 
structions, among  the  most  remarkable  of 
these  being  the  bridge  of  Chatellerault, 
460  ft.  long,  comprising  three  arches,  two 
of  133  ft.  span  and  one  of  167  ft. 

Hennebique's  first  patent  dates  from  1892 
(British  patent,  No.  14,530),  and  in  this  he 
demonstrates  the  utility  of  stirrups  to  rein- 
force beams  against  shear,  in  which  matters 
he  had  to  an  extent  been  anticipated  by 
Hyatt  in  1877  and  Meyenberg  in  1891.  In 
1897  Hennebique  introduced  cranked-up 
rods,  and  placed  these  one  above  the  other, 
so  as  to  reduce  the  width  of  the  beam, 
following  (to  some  extent)  the  lines  laid 
down  by  Hyatt  in  1877  and  F.  G.  Edwards 
in  1892,  in  which  latter  year  M.  Koenen  and 
G.  A.  Wayss,  of  Germany,  patented  in 
England  a  method  of  floor  construction  with 
rods  cranked  up  at  the  point  of  contra- 


flexure,  "  the  parts  in  tension  being  strength- 
ened by  roughened  or  serrated  metal  rods 
or  strips  embedded  in  the  structure." 

Progress  since  1890. — Speaking  of  the 
period  following  the  years  1890  to  1892, 
N.  de  Tedesco,  Ingenieur  des  Arts  et 
Manufactures,  and  editor  of  Lz  Citnent,  has 
remarked  that  the  modern  method  of  build- 
ing remained  for  a  long  time  in  very  few 
hands,  and  nothing  was  to  be  found  in 
literature  concerning  the  calculation  of 
structures  constituted  •  by  two  different 
materials.  Contractors  were  obliged  to  ex- 
periment in  order  to  ascertain  the  most 
suitable  proportions  and  arrangements  of 
these  materials  for  resisting  the  determinate 
load  on  a  determinate  span,  but  they  were 
scarcely  able  to  undertake  with  advantage 
investigations  of  that  kind.  Such  work  was 
rather  the  role  of  "ingenieurs  des  ponts  et 
chaussees,"  but  many  of  these  attempted  to 
co-ordinate  the  results  of  the  tests  carried 
out  by  Coignet  and  Hennebique,  only  to 
find  that  it  was  impossible  to  deduce  mechan- 
ical laws  from  them,  inasmuch  as  the 
mechanical  properties  of  concrete  are  vari- 
able with  the  quality  of  sand  or  gravel,  the 
cement  used,  the  degree  of  fluidity  of  the 
mixture,  the  climatic  influences  during  the 
hardening  process,  the  age,  and  with  many 
other  surrounding  conditions.  Considere 
came  to  the  rescue,  and  by  a  series  of  tests 
undertaken  upon  small  specimens  manu- 
factured under  uniform  conditions  he  was 
able  to  discover  the  laws  governing  the 
deformation  of  concrete  submitted  to  stress. 
In  1890  Paul  Neumann,  in  a  published 
memoir,  described  his  mode  of  calculation, 
which  recognised  the  relation  between  the 
coefficients  of  elasticity  of  the  two  materials, 
steel  and  concrete  ;  and  about  the  same 
time  T.  Melan  published  a  treatise  in  which 
he  came  to  practically  the  same  conclusion 
as  Neumann — namely,  that  the  elastic  be- 
haviour of  concrete  was  not  the  same  under 
compression  as  it  was  under  tension.  Melan, 
by  the  way,  invented  in  1890  the  system  of 
constructing  floors  and  vaulting  named 
after  him.  The  first  important  contribu- 
tion to  a  rational  theory  was  the  memoir 
written  by  Edmond  Coignet  and  N.  de 
Tedesco,  and  published  in  the  Bulletin  of  the 
Societe  des  Ingenieurs  Civils  of  France  for 
March,  1894.  Into  Holland  and  Denmark, 
reinforced  concrete  was  introduced  about 
1890,  the  floors  of  the  National  Gallery 
at  Copenhagen  being  built  of  reinforced 


IO 


REINFORCED    CONCRETE 


concrete  in  1891,  the  same  system  being 
used  in  the  next  year  or  so  for  har- 
bour works,  and  in  1894  for  a  19-metre 
span  bridge  for  foot  passengers,  just  out- 
side Copenhagen.  In  Switzerland,  where 
reinforced  concrete  had  previously  been  used 
for  vaulting,  1892  saw  the  introduction 
of  Hennebique  beams  subjected  to  bending 
stress.  Prof.  F.  Schule,  of  Zurich,  has 
pointed  out  that  two  conditions  favour 
the  development  of  the  new  mode  of  con- 
struction in  Switzerland :  first,  Switzerland 
has  to  import  all  its  steel,  and  reinforced 
concrete  involves  a  saving  in  this  material 
not  to  be  undervalued  ;  and,  secondly,  the 
excellent  position  occupied  by  Switzerland 
with  regard  to  the  manufacture  of  port- 
land  cement.  Both  in  Hungary  and  Italy, 
reinforced  concrete  was  now  making  rapid 
progress,  and  by  1896  a  bridge  having  a 
span  of  more  than  25  metres  was  being 
erected  at  Sarajevo,  in  Bosnia.  In  1897  the 
Hennebique  system  was  introduced  into 
Great  Britain  by  (the  late)  L.  G.  Mouchel, 
Francois  Hennebique's  partner,  and  since 
then  its  progress  in  this  country  has  been 
truly  remarkable. 

Coming  to  the  Paris  International  Exhibi- 
tion of  1900,  the  graceful  palaces  built  by 
Edmond  Coignet  and  Francois  Hennebique 
attracted  the  favourable  notice  of  architects 
and  engineers  the  wide  world  over,  and 
thousands  of  illustrated  papers  throughout 
the  Continents  published  photographs  of  the 
Chateau  d'Eau,  erected  by  Coignet.  If  there 
was  anything  necessary  to  stimulate  the 
adoption  of  reinforced  concrete  design  by 
the  engineers  and  architects  of  the  day,  it 
was  afforded  by  this  advertisement,  from 
which  dates  an  interest  in  the  subject  out 
of  all  proportion  to  that  formerly  existing. 

Of  the  few  further  inventions  which  it  is 
worth  specially  to  note  may  be  mentioned 
the  Siegwart  beam,  introduced  in  Switzer- 
land in  the  year  1902.  The  inventor,  a 
Lucerne  architect,  desired  to  avoid  con- 
struction on  the  building  site  other  than 
the  actual  laying  of  the  prepared  beam,  and 
his  solution  of  the  problem  is  satisfactory 
where  the  span  does  not  exceed  about 
5  metres.  The  Visintini  system,  invented 
in  Austria  and  patented  in  Great  Britain  in 
1902,  introduces  framed  beams  into  the 
reinforced  concrete  construction,  and  it  came 
into  practical  employment  in  1903,  later 
coming  into  use  for  the  vaulting  of  the 
Evangelical  Church  at  Aussig. 


A  specification  of  particular  interest  is 
No.  24,371  of  1904,  the  first  British  patent 
granted  to  Edmond  Coignet.  It  describes 
and  illustrates  beams,  floors,  domes,  roofs, 
columns,  and  walls,  and  has  a  high  educa- 
tional value.  The  drawings  of  beams  show 
the  tension  bars  in  the  lower  part  of  the  beam 
to  be  connected  to  the  compression  bars  in 
the  upper  part  by  means  of  ties.  These  ties 
also  hook  over  the  tension  bars,  which  extend 
through  the  floor  slabs  and  which  are  bent 
up  to  pass  over  the  compression  rods. 

Notable  American  Inventions. — As  a 
result  of  the  rare  commercial  ability  behind 
them,  certain  American  systems  have  be- 
come extremely  well  known  in  Great  Britain, 
and  perhaps  in  consequence  of  this  fact 
there  is  a  common  tendency  to  look  upon  the 
United  States  as  the  real  "  home "  of 
reinforced  concrete.  While  there  is  no 
doubt  that  the  temperament  of  the  Americans 
led  them  to  welcome  the  "  new  "  method 
of  building  (as,  indeed,  they  welcome  any 
promising  invention),  as  a  matter  of  fact, 
there  is  not,  prior  to  1892,  much  that  is  of 
truly  American  origin  and  of  real  note  in 
the  United  States  records  with  regard  to 
reinforced  concrete  excepting  the  Thaddeus 
Hyatt  inventions  and  the  Ransome  twisted 
bar,  to  both  of  which  reference  has  already 
been  made.  In  1892,  August  C.  Storck,  of 
St.  Louis,  patented  a  method  of  strengthen- 
ing a  concrete  block  with  an  embedded 
"  brace  "  of  slashed  (expanded)  metal ;  and, 
in  the  following  year,  Thomas  A.  Lee,  of 
Kansas  City,  filed  a  specification  for  a  "  non- 
shearing  rod  for  strengthening  concrete," 
two  or  more  rods  to  be  laid  together  spirally  ; 
and  E.  L.  Ransome  took  out  what  is  appar- 
ently his  first  patent  for  a  definite  method  of 
reinforcing  concrete,  his  specification  describ- 
ing and  illustrating  a  slab  supported  at  its 
ends  by  walls  and  reinforced  by  two  bars,  a 
shorter  one  above  a  longer  one,  the  bars 
tapering  to  nothing  at  each  end. 

Some  years  previous  to  this,  Ransome  had 
erected  some  important  buildings  in  rein- 
forced concrete. 

In  1896,  Edwin  Thacher,  of  Detroit,  filed 
an  interesting  specification  showing  a  con- 
crete arch  between  abutments,  the  arch  being 
reinforced  with  two  series  of  flat  metal  bars 
in  pairs,  one  bar  of  each  pair  above  the 
other.  At  intervals,  the  bars  had  pegs  which 
projected  on  each  side,  apparently  to  aid 
adhesion.  Ira  A.  Shaler,  of  New  York, 
patented  in  1900  a  method  of  reinforcing 


HISTORICAL    NOTES 


ii 


slabs  by  means  of  a  metal  skeleton  comprising 
longitudinal  and  cross  members,  the  one 
welded  to  the  other.  Then  came,  in  1901, 
two  important  E.  L.  Eansome  patents  ;  in 
the  earlier,  webs  of  hardened  reinforced  con- 
crete are  placed  in  position,  the  spaces  be- 
tween them  spanned  with  falsework,  and 
the  top  compression  member  of  the  floor 
then  made  with  concrete ;  in  the  later,  a 
reinforced  concrete  floor  extends  to  the 
exterior  face  of  a  building,  and  there  becomes 
a  belt  course  which  caps  the  piers,  a  down- 
ward extension  forming  heads  or  lintels  to 
the  windows  below.  From  this  date  the 
inventions  become  too  numerous  even  to 


mention,  but  space  must  be  found  for  a 
reference  to  the  first  Kahn  patent,  filed  on 
December  11,  1902,  and  bearing  date  of 
August  18,  1903.  It  describes  the  now 
well-known  Kahn  bar,  which  has  members 
projecting  obliquely  so  as  to  form  the 
diagonal  members  of  a  part  of  a  truss.  An 
improvement  followed  in  a  patent  filed  on 
May  4,  1903,  and  bearing  date  November 
3,  1903.  These  were  the  forerunners  of 
a  large  number  of  patents  by  the  same 
inventor,  the  assignees  of  the  patents  being 
the  Trussed  Concrete  Steel  Company,  now 
well  known  both  in  the  United  States  and 
in  England. 


Concrete :    Materials,   Proportions 

and   Mixing 


What  Concrete  is. — Concrete  is  an  arti- 
ficial stone  made  by  cementing  together 
fragments  of  hard  material.  The  hard 
material,  such  as  brick,  stone,  etc.,  is  known 
as  the  "  aggregate "  (literally,  something 
that  has  been  gathered  together)  ;  the 
cementing  material,  such  as  lime  or  cement, 
is  termed  the  "  matrix  "  (literally,  but,  in  the 
case  of  concrete,  not  actually,  a  mould  in 
which  something  is  embedded).  The  active 
agents  in  the  process  of  solidifying  are  the 
cementing  material  and  the  water ;  the 
aggregate  is  inactive. 

Making  a  Strong  Concrete.  —  The 
strength  and  quality  of  concrete  depend 
chiefly  upon  (1)  the  nature  of  the  aggregate  ; 
(2)  the  nature  of  the  matrix ;  (3)  the  pro- 
portions in  which  the  materials  (including 
water)  are  mixed  together ;  and  (4)  the 
efficiency  of  the  mixing  process. 

The  compressive  strength  of  concrete  is 
limited  by  that  of  the  aggregate,  and  the 
tensile  strength  cannot  exceed  that  of  the 
cement  bond.  The  ideal  concrete  can  be 
pictured  as  a  composite  block  of  stone, 
brick,  etc.,  in  which  all  the  pieces  fit  together 
perfectly,  being  attached  one  to  the  other 
by  a  film  of  cement.  Such  concrete  is  not 
attainable  in  practice,  the  shapes  of  the 
fragments  preventing  such  extremely  close 
fitting,  and  thus  causing  the  formation  of 
pockets,  which  become  filled  with  a  mixture 
of  cement  and  sand.  Better  concrete  than 
is  generally  made  would  result  from  the  use 
of  graded  aggregates  ;  first,  a  very  coarse 
aggregate  would  be  selected,  then  a  much 
finer  aggregate  to  occupy  the  larger  of  the 
voids,  then  a  still  finer  one  to  occupy  the 
lesser  voids  in  the  aggregate,  and  finally 
the  cement.  According  to  much  of  the 
present  practice,  there  are  but  two  grades 
in  the  aggregate,  the  coarse  (stones,  gravel, 
etc.)  and  the  fine  (sand). 

A  little  thought  will  show  that  the  strength 
of  concrete  may  be  affected  unfavourably 
by  any  one  of  a  number  of  conditions  :  Any 
pait  of  the  surface  of  the  aggregate  not 
covered  with  cement ;  any  grain  of  sand 


without  a  cement  coat ;  any  coarse  particle 
of  cement  which  may  in  the  future  undergo 
chemical  change  ;  any  defective  particle  of 
cement  that  was  incapable  of  doing  its  duty 
when  brought  into  contact  with  moisture 
in  course  of  mixing  ;  any  very  fine  dust 
from  the  aggregate,  inert  and  useless  ;  any 
crack  or  fissure  in  the  broken  stone ;  any 
hollow  place  resulting  from  insufficient  tamp- 
ing, etc.  Any  one  of  these  will  constitute 
a  source  of  weakness. 

Concrete-making  materials  will  now  be 
considered  in  detail.  In  preparing  the 
information  here  presented  on  aggregates, 
care  has  been  taken  to  make  it  conform  to 
a  schedule  issued  with  the  "  Interim  Keport  " 
of  the  Special  Commission  on  Concrete 
Aggregates  appointed  by  the  British  Fire 
Prevention  Committee. 

NATURAL   AGGREGATES 

Gravel. — This  usually  consists  of  smooth, 
round  pebbles  with  adherent  sand,  clay, 
and  carbonaceous  matter.  The  large  stones 
need  to  be  broken,  and  the  sand  to  be  sifted. 
Clayey  gravel  must  be  washed,  as  raw  clay 
seriously  affects  the  strength  of  the  concrete 
and  retards  the  setting  of  the  cement.  It  is 
generally  wise,  as  a  matter  of  fact,  to  wagh 
all  gravel.  Soft  or  shaly  should  be  avoided. 

1  cub.  ft.  of  coarse  gravel  usually  weighs 
about  97  lb.,  and  clean  shingle  about  93  Ib. 

Shingle. — This  is  a  kind  of  gravel  found 
on  the  seashore,  its  particles  being  of  a 
rounded  shape.  It  may  contain  a  small  but 
significant  proportion  of  absorbed  free  salts, 
not  readily  removed  by  washing.  If  coarse 
material,  free  from  sand,  is  required,  it  is 
best  to  specify,  "  clean  washed  and  broken 
shingle." 

Sandstone. — This  consists  of  grains  of 
sand,  usually  quartz,  held  together  by  a 
natural  cement,  the  nature  of  which  may 
vary  in  different  examples.  It  is  far  from 
being  the  best  of  the  stone  aggregates,  owing 
to  its  absorbent  nature,  and  its  weakness 
when  wetted.  It  cannot  withstand  abrasion, 
and,  as  a  rule,  is  of  doubtful  value  as  a 


CONCRETE:    MATERIALS   AND    MIXING 


fire  resistant.  When  it  must  be  used,  care 
should  be  taken  that  it  is  dense,  uniform, 
and  as  homogeneous  as  possible  in  struc- 
ture. The  Interim  Report  before  alluded 
to  states  that  the  weights  of  sandstones, 
limestones,  quartzites,  and  rocks  of  similar 
character  should  not  be  less  than  130  Ib. 
per  cubic  foot,  the  crushing  strength  not  less 
than  about  3,080  Ib.  per  square  inch,  and  the 
water  absorption  not  more  than  8  per  cent, 
of  the  weight  of  the  stone  after  a  24-hour 
immersion.  The  aggregate  after  preparation 
should  be  free  from  all  dirt,  decomposed  rock, 
clay  and  organic  material. 

Limestone. — As  in  the  case  of  sandstone, 
the  quality  varies.  The  oolites,  when  of 
fine  grain,  answer  very  well  as  a  concrete 
aggregate,  especially  certain  crystalline  lime- 
stones of  the  magnesian  or  dolomite  series. 
Limestone  consists  mainly  of  calcium  car- 
bonate (with  or  without  magnesium  car- 
bonate), deposited  originally  from  water,  or, 
in  any  case,  formed  through  the  agency  of 
water.  It  should  have  the  qualifications 
mentioned  in  the  "  sandstone  "  paragraph 
as  regards  weight  and  strength.  An  average 
weight  is  about  168  Ib.  per  cubic  foot,  and 
the  average  strength  under  compression  about 
4,700  Ib.  per  square  inch.  Limestones  are 
dangerous  in  case  of  fire,  becoming  converted 
into  lime. 

Limestone,  sandstone,  etc.,  are  sometimes 
used  in  a  crushed  form  unscreened  so  that 
the  very  fine  pieces  and  the  dust  take  the 
place  of  sand,  which  can  therefore  be  omitted. 
There  is,  however,  a  strong  objection  to  the 
use  of  this  material.  No  flour-like  dust, 
except  that  of  cement,  ought  to  be  incor- 
porated into  concrete.  Stone  dust  is  quite 
inert,  and  should  it  collect,  as  is  likely,  in 
the  form  of  small  lumps  throughout  the 
mass,  it  will  constitute  a  source  of  weakness. 

Argillaceous  or  clayey  limestones  and  the 
softer  and  more  shaly  limestones  are  un- 
desirable aggregates. 

Chalk. — This  is  a  variety  of  limestone,  and 
varies  in  hardness,  the  upper  strata  being 
soft,  while  the  middle  and  lower  strata  are 
much  harder,  and  make  better  concrete  than 
some  other  materials.  Even  the  top  stratum 
forms  a  fairly  good  concrete  for  walls  where 
the  height  is  inconsiderable.  It  has  been  used 
for  two-storey  cottages  having  suspended 
concrete  floors  with  satisfactory  results,  being 
found  to  resist  frost  effectually  and  to 
possess  a  considerable  crushing  stress.  The 
fine  sandy  portions  should  be  eliminated  by 


screening  and  coarse,  sharp  pit  or  river  sand 
or  fine  brick  sand  or  shingle  substituted.  In 
view  of  the  large  chalk  area  in  the  south  of 
England,  and  where  better  materials  are 
often  not  available,  it  might  be  used  to 
advantage  for  walls  of  buildings  where 
economy  in  construction  is  desirable. 

Quartzite.  —  This  in  many  respects  re- 
sembles sandstone,  but  is  harder  and  denser, 
resists  abrasion  better,  is  not  so  absorbent, 
and  is  therefore  a  better  aggregate.  It 
actually  is  a  sandstone  which  has  become 
hardened  by  natural  forces  and  heat. 

Flint. — This  very  hard  but  brittle  stone 
consists  of  grey  or  black  siliceous  matter,  and 
its  probable  source  was  a  tiny  sponge  around 
which  colloidal  silica  has  collected  and  coagu- 
lated from  the  surrounding  water,  thus  form- 
ing the  peculiar  shapes  in  which  flint  occurs. 
Flints  are  generally  found  in  bands  in  one 
of  the  chalk  series,  but  they  also  occur  on 
the  seashores,  where  they  have  been  deposited 
after  the  erosion  of  the  chalk  beds. 

Flints  obtained  from  the  surface  of 
arable  land  are  somewhat  dirty  and  re- 
quire washing,  which  is  more  easily  per- 
formed after  they  have  been  broken  or 
crushed.  Flint  cannot  be  broken  by  hand 
to  make  a  suitable  aggregate,  but  should  be 
crushed  by  a  stone  breaker. 

Flint  weighs  about  162  Ib.  per  cubic 
foot,  and  the  compressive  strength  is  high, 
about  7,800  Ib.  per  square  inch.  Flint  is 
liable  to  fly  when  subjected  to  heat,  but  the 
tendency  is  reduced  by  crushing  to  small 
particles  less  than  f  in.  in  size.  It  is  not 
absorbent,  and  is  not  durable  under  shock. 
Shingle  is  largely  water-worn  flint. 

Granite, — This  is  an  igneous  rock  of 
greater  strength  under  compression  (exceed- 
ing 16,000  Ib.  per  sq.  in.)  and  of  varying 
specific  gravity,  an  average  specimen  weigh- 
ing about  170  Ib.  per  cubic  foot. 

Granite  is  an  excellent  natural  aggregate 
providing  that  it  is  free  from  appreciable 
quantities  of  weathered  or  partially  decom- 
posed material.  The  chief  objection  to  its 
use  is  its  high  specific  gravity,  which  tends 
to  make  it  settle  out  of  the  concrete  when 
the  latter  is  being  placed  in  position,  but 
with  a  fairly  stiff  concrete  the  danger  of 
settlement  is  small.  The  granite  must  be 
crushed  to  pieces  of  suitable  size  and  sifted 
free  from  sandy  and  dust-like  particles  by 
screening  over  a  J-in.  mesh.  The  granite 
should  be  well  watered  immediately  before 
use,  as  some  varieties  are  highly  porous  and, 


REINFORCED    CONCRETE 


if  used  dry,  greatly  reduce  the  strength  of 
the  concrete.  If  the  watering  is  effected  by 
means  of  a  hose  it  will  incidentally  wash 
off  any  loosely  adherent  material,  the  pre- 
sence of  which  is  detrimental  to  the  concrete. 

The  value  of  granite  depends  on  the  sharp- 
ness and  angularity  of  the  fragments  as  well 
as  on  the  roughness  of  the  surface.  Those 
granites  should,  therefore,  be  chosen  that 
are  naturally  brittle,  and  those  which  pro- 
duce rounded  fragments  in  a  crusher  should 
be  avoided  as  far  as  possible.  The  maximum 
size  of  granite  particles  is  usually  taken  at 
|  in.,  but  for  very  large  blocks  fragments  up 
to  2 \  in.  in  diameter  are  used. 

There  is  little  difference  in  the  quality  of 
granites  from  various  sources,  so  far  as  their 
value  as  aggregates  is  concerned,  so  that 
the  chief  attention  should  be  paid  to  select- 
ing a  granite  of  clean  appearance,  with  as 
rough  a  surface  as  possible  (to  give  a  good 
key  to  the  cement),  and  one  of  which  the 
fragments  are  highly  angular.  Naturally, 
where  a  granite  can  be  obtained  locally,  that 
will  be  the  granite  selected  for  a  particular 
case. 

Other  Volcanic  Rocks.  —  These  include 
basalts,  lavas,  pumice,  etc.  They  should  be 
free  from  soft  portions,  and  should  be  tested 
by  immersion  in  water  for  72  hours,  at 
the  end  of  which  time  there  should  be  no 
signs  of  expansion,  disintegration,  or  solu- 
tion. Basalts,  traps,  dense  lavas,  etc., 
should  have  a  thoroughly  vitrified  structure, 
be  homogeneous,  and  show  a  clean  fracture. 
The  basalts  make  really  good  aggregate, 
being  strong  (compressive  strength,  about 
19,000  Ib.  per  square  inch) ;  they  are  durable 
under  abrasion,  and  they  are  of  low  water 
absorption ;  but  they  have  two  disadvantages 
— they  are  rather  heavy  (180  Ib.  per  cubic 
foot),  and  they  have  no  marked  resistance 
to  fire  unless  care  is  taken  to  break  them 
up  small.  Not  so  hard  as  the  basalts,  traps, 
etc.,  are  the  lavas,  including  pumice,  which, 
however,  for  use  as  aggregates,  should  be 
moderately  hard,  show  a  bright,  silky  frac- 
ture and  be  free  from  dust  and  impurities. 

ARTIFICIAL   AGGREGATES 

Burnt  Clay. — Under  this  sub -heading 
may  be  included  broken  bricks,  clay  ballast, 
broken  terra-cotta,  broken  pottery,  etc. 
The  broken  bricks  should  be  such  as,  if 
whole,  would  be  suitable  for  building  pur- 
poses ;  all  soft  and  under-burned  portions, 
should  be  discarded.  Broken  bricks  should 


be  free  from  old  mortar,  and  from  dust 
that  will  pass  through  a  J-in.  mesh.  Most 
broken  bricks  are  very  absorbent,  and  for 
this  reason  should  be  watered  before  mixing 
with  the  other  ingredients,  as  otherwise  the 
cement  is  robbed  of  moisture  and  the  setting 
is  injuriously  affected.  Burnt  clay  ballast 
always  needs  to  be  inspected  carefully  for 
unburnt  particles,  and  it  is  best  to  test  a 
sample  by  soaking  in  water  for  a  few  days 
to  see  whether  it  disintegrates  or  not ;  it 
needs  watering  in  the  same  way  as  the 
broken  brick.  Porous  terra-cotta  and  un- 
glazed  earthenware  make  good  aggregate, 
and  these  also  should  be  watered  before  use  ; 
the  material  should  be  clean,  hard,  and  well 
burnt,  but  any  large  flat  pieces  or  pieces 
of  distinctly  curved  shape  should  be  regarded 
with  suspicion,  since  they  do  not  work  in  well 
with  the  other  pieces,  and,  by  "bridging," 
may  easily  form  cavities. 

Residues. — These  include  blast-furnace 
slag,  clinker,  coke  breeze,  and  cinders,  which 
have  been  proved  valuable  for  special  appli- 
cations, being  light  and  fire-resisting ;  but 
it  is  of  the  utmost  importance  that  sulphur, 
if  present  in  anything  more  than  a  mere 
trace,  be  eliminated,  as  otherwise  it  tends  to 
combine  with  the  cement  during  the  process 
of  setting  and  seriously  affects  its  strength. 
Then,  too,  any  dense  combustible  material 
present  (as  coal)  is  a  defect  which  militates 
against  the  successful  use  of  residues. 

If  materials  containing  sulphur  are  exposed 
to  the  atmosphere  for  a  time  and  moved 
occasionally,  the  sulphur  tends  to  disperse. 
The  smell  of  the  materials  when  moved  is 
the  simplest  evidence  of  the  presence  of 
sulphur. 

Blast-furnace  Slag. — This  should  come 
from  pig-iron  smelting  furnaces,  and  basic 
slag  should  be  rejected.  If  there  is  any 
doubt  as  to  the  sulphur  content,  and  in 
cases  where  the  source  of  the  slag  is  known, 
the  works  chemist  should  be  asked  for  the 
analysis,  which  in  many  cases  he  should 
be  able  to  supply  without  much  trouble. 
Only  a  mere  trace  of  sulphur  is  permissible, 
and  the  slag  should  be  washed  to  remove 
dust  and  possibly  some  of  the  sulphur. 
Washing  will  also  remove  any  free  lime. 

Coke  Breeze. — The  Interim  Keport,  of 
which  mention  has  already  been  made, 
recommends  that  coke  breeze  for  use  as 
aggregate  shall  be  entirely  coke  taken  from 
gas  retorts,  coke  ovens  or  special  furnaces, 
and  be  absolutely  free  from  clinker,  coal, 


CONCRETE:    MATERIALS    AND    MIXING 


and  all  substances  that  will  not  float  in 
water,  and  from  any  admixture  of  material 
taken  from  the  retort  furnace  or  water  pan 
below  it,  and  from  cinder,  ash,  or  other 
adulterant.  On  no  other  aggregate  is 
expert  opinion  divided  to  such  an  extent  as 
on  coke  breeze.  All  agree  that  its  light- 
ness makes  it  particularly  adapted  to  sus- 
pended floors,  and  it  has  another  advantage 
— nails  can  be  driven  into  it ;  but  it  lacks 
strength.  Its  weight  is  about  35  Ib.  per 
cubic  foot.  Being  so  very  porous,  it  might 
be  thought  highly  absorbent,  but  such  is 
not  the  case,  as  many  of  the  cavities  formed 
by  the  bubbles  of  gas  evolved  when  the 
coal  is  heated  are  sealed  and  therefore  have 
no  communication  with  the  outside,  and 
it  is  therefore  only  the  surface  (and 
broken)  cavities  into  which  the  water  can 
penetrate.  Some  coke  breeze  has  so  high 
a  content  of  sulphur  that  its  use  is  a  posi- 
tive source  of  danger,  but  where  this  is  not 
the  case,  the  material  is  a  valuable  one. 
Much  depends  upon  the  kind  of  coal  of 
which  the  breeze  is  the  residue.  Thomas 
Potter,  in  a  paper  read  before  the  Concrete 
Institute,  stated  that  he  had  used  thousands 
of  tons  of  coke  breeze  for  floors,  and  did  not 
remember  a  failure  of  any  kind,  and  that  he 
had  proved  it  to  be  one  of  the  best  materials 
for  floors  and  roofs.  This,  however,  is  not 
the  experience  of  all  users  of  the  material,  but 
this  may  be  due  to  sifted  ashes  ("  builders' 
breeze")  having  been  substituted  for  true 
coke  ashes. 

Clinker,  Cinders,  etc. — Clinker  used  for 
concrete  should  be  the  thoroughly  burnt 
and  hard  waste  product  of  furnaces,  free 
from  dust,  shale,  or  free  lime.  The  quality 
of  clinker,  cinders,  etc.,  varies  greatly,  but 
no  material  containing  ashes,  dust,  rubbish, 
and  more  than  a  trace  of  sulphur  is  suit- 
able as  an  aggregate.  In  a  later  chapter 
(p.  261),  the  results  of  some  fire  tests  of 
cinder  concrete  are  given,  and  their  some- 
what contradictory  nature  will  be  noted. 

Broken  Concrete. — Old  and  very  hard 
concrete  may  be  broken  up  so  as  to  pass  a 
H-in.  ring,  well  screened  in  order  to  remove 
all  dust,  and  then  used  as  the  aggregate  in 
concrete  for  foundation  work  or  rough 
walling  ;  but  the  material  would  not  be 
sufficiently  reliable  for  floors  or  fine  work. 

On  no  account  should  old  concrete  in 
which  calcium  sulphate  or  plaster-of-paris 
was  the  matrix,  as  was  the  general  practice 
before  portland  cement  was  used,  be  utilised 


as  an  aggregate,  because  when  water  is 
added  it  expands  and  ruptures  the  concrete, 
no  matter  how  long  since  it  was  origin- 
ally employed.  Failures  of  the  kind  have 
occurred  where  the  plaster  concrete  was 
forty  years  old. 

SIZE  OF  AGGREGATE 

In  no  work  ought  the  stones  to  be  greater 
than  2J  in.  or  3  in.  across,  this  size  being 
used  for  large  retaining  walls,  foundations, 
etc.  For  walls,  the  stones  should  pass  a 
2-in.  mesh  and  be  retained  on  a  1-in.  sieve  ; 
and  for  floors  they  should  pass  a  f-in.  mesh 
and  be  retained  on  a  ^-in.  one.  For  the 
surface  of  concrete  floors  the  stones  should 
be  broken  so  as  to  pass  a  f  in.  or  \  in.  mesh. 

The  larger  the  stones,  the  stronger  will 
be  the  concrete,  the  quantity  of  cement 
remaining  constant.  According  to  experi- 
ments undertaken  by  Messrs.  Fuller  and 
Thompson,  concretes  with  stones  2£  in.,  1  in., 
and  \  in.  in  diameter  respectively  required 
cement  in  the  relative  proportions  of  6,  7, 
and  8  to  be  of  the  same  strengths.  The 
experiments  proved,  too,  that  round,  water- 
worn  material,  such  as  gravel,  gave  a  denser 
mass  than  broken  stone,  but  that  the  latter 
gave  the  stronger  concrete.  The  size  of  the 
aggregate  particles  greatly  affects  the  pro- 
portioning of  the  ingredients. 

WASHING  COARSE  AGGREGATE 

It  is  nearly  always  desirable  to  wash  the 
aggregate,  and  the  work  should  be  done  on 
a  wooden  platform,  inclined  somewhat  to 
allow  water  to  run  off  freely.  In  preparing 
fairly  large  quantities  for  use,  two  men 
should  shovel  the  material  backwards  and 
forwards,  while  a  third  keeps  a  constant 
supply  of  water  playing  on  the  aggregate, 
this  being  continued  till  the  water  runs  off 
clear.  The  use  of  a  mechanical  washer — 
some  form  of  tower  or  tumbling  drum — gives 
better  results.  For  good  concrete  the  im- 
portance of  washing  the  aggregate  cannot 
be  over-estimated.  The  water  leaves  the 
material  in  the  proper  condition  for  causing 
the  cement  to  adhere  closely  to  it,  and  the 
dampness  undoubtedly  assists  the  setting. 

SAND 

There  appears  to  be  a  difference  of  opinion 
as  to  the  function  of  sand  in  concrete. 
Some  regard  it  as  a  cheapener  to  be  added 
to  the  cement  to  increase  its  bulk  and  form 
a  mortar  in  which  the  coarse  aggregate  is 


i6 


REINFORCED    CONCRETE 


embedded.  The  writer  does  not  share  that 
view.  The  sand  is  essentially  the  fine  part 
of  the  aggregate,  and  the  proportion  of  it 
to  be  used  depends  not  on  the  amount  of 
cement  thought  to  be  necessary,  but  on  the 
percentage  of  voids  in  the  coarser  aggregate. 
Then,  in  turn,  the  proportion  of  cement 
will  depend  on  the  percentage  of  voids  in 
the  sand  and  between  the  sand  and  the 
coarser  aggregate,  there  being  in  addition 
enough  to  coat  the  whole  of  the  aggregate. 
But  for  the  trouble,  sand  of  two  sizes  could 
be  used,  the  finer  occupying  the  voids  in 
the  larger.  It  is  significant  that  experi- 
ments carried  out  by  Messrs.  Fuller  and 
Thompson  seem  to  prove  that  the  strength 
and  density  of  concrete  are  affected  by 
variation  in  the  size  of  sand  grains,  and 
that  an  excess  of  fine  or  medium  sand 
decreases  both  strength  and  density. 

Sand  must  be  sharp  and  gritty,  and 
washed  thoroughly  free  from  everything 
that  does  not  consist  of  small  particles  of 
stone,  quartz,  or  non-plastic  material.  Dust, 
clay,  earth,  vegetable  matter,  and  all  other 
plastic  substances,  are  sources  of  weakness, 
and  the  necessity  of  eliminating  them  invests 
with  the  greatest  importance  the  washing 
and  screening  of  pit  sand.  Kiver  sand  may 
not  need  washing,  but  is  improved  by  screen- 
ing. Sea  sand  should  have  the  salt  washed 
out.  Pit  sand,  when  of  glacial  origin,  is 
unsuitable ;  so  is  sand  containing  more  than 
10  per  cent,  of  very  fine  grains. 

1  cubic  foot  of  pit  sand  weighs  about 
102  Ib.  ;  river  sand,  106  Ib.  ;  coarse  gravel, 
97  Ib. ;  and  clean  shingle,  93  Ib. 

Substitutes  for  Natural  Sand. — Where 
sand  is  difficult  to  obtain,  a  substitute  may 
be  used.  Indeed,  the  crushed  stone  sub- 
stitutes, washed  perfectly  free  from  dust, 
give  a  stronger  concrete  than  do  some  sands, 
because  the  artificial  crushing,  when  properly 
carried  out,  produces  very  sharp  and  angular 
particles.  In  addition  to  crushed  stone, 
sand  substitutes  include  ground  burnt  clay 
(pottery,  bricks,  etc.),  slag  sand  (made  by 
running  molten  slag  into  water  whereby  it 
becomes  granulated),  etc.  All  these  need  to 
be  washed  or  screened  to  remove  flour-like 
dust,  because  any  material  that  passes 
through  a  sieve  of  50  meshes  to  the  linear 
inch  is  too  fine  to  be  satisfactorily  used. 

Washing  Sand. — The  best  way  to  wash 
sand  is  in  a  stream  of  water,  the  rate  of 
flow  of  which  is  adjusted  to  remove  the  par- 
ticles of  clav,  silt  and  rock-flour  without 


carrying  of!  the  sand.  Some  mechanical 
arrangement  for  keeping  the  sand  in  motion 
and  to  prevent  the  formation  of  eddies  is 
necessary,  or  some  portions  will  not  be  well 
washed.  One  of  the  most  effective  sand- 
washers  consists  of  a  large  inclined  pipe 
about  15  ft.  in  length,  in  which  revolves  a 
screw  conveyor.  A  hopper  is  fitted  to  the 
lower  end  of  the  pipe,  and  through  this  the 
dirty  sand  is  supplied,  together  with  the 
necessary  water.  The  water  flows  out  at 
the  upper  end  of  the  pipe  carrying  the  clay, 
silt  and  rock-flour  with  it,  and  leaving  the 
washed  sand  behind.  As  soon  as  the  water 
flows  away  in  a  clear  stream  the  water  supply 
is  cut  off  and  the  sand  removed.  This 
arrangement  has  been  made  continuous  in 
action,  and  greatly  improved  by  the  addition 
of  a  second  pipe  about  half-way  along  the 
larger  one  ;  this  second  pipe  carries  off  the 
washed  sand,  and  delivers  it  in  a  very  wet 
state  into  wagons  or  on  to  a  receiving  bed. 
Such  an  arrangement  has  been  proved  to  use 
much  less  water  in  proportion  to  sand  than 
the  ordinary  cylindrical  washers  generally 
employed. 

The  use  of  sieves  for  washing  sand  is  con- 
venient for  quantities  of  not  more  than 
3  to  5  cwt.,  but  is  too  slow  and  costly  for 
larger  amounts.  The  sand  to  be  washed  is 
laid  on  the  sieve  so  as  to  cover  to  a  thickness 
of  not  more  than  an  inch,  and  the  sieve  is 
then  partly  immersed  in  a  tub  of  water  and 
shaken  to  and  fro.  The  idea  is  that  the 
finer  particles  will  pass  through  the  sieve 
and  fall  into  the  water  beneath,  but  an 
examination  of  sand  treated  in  this  manner 
will  show  that  it  contains  a  considerable 
quantity  of  fine  mud,  which  is  too  adhesive 
and  remains  suspended  too  readily  in  water 
to  be  removed  by  this  treatment.  Another 
method  of  sieve-washing  consists  in  choosing 
a  sieve  with  a  rather  coarser  mesh  than 
corresponds  with  the  sand,  and  then  washing 
the  sand  through  the  sieve  into  a  tub  of 
water.  The  coarse  material  remains  on  the 
sieve,  and  it  is  supposed  that  the  clay  and 
silt  will  be  removed  when  the  water  is  poured 
slowly  out  of  the  tub.  An  examination  of 
the  sand  left  will  show,  however,  that  it 
contains  a  considerable  proportion  of  adhe- 
rent clay  and  rock-flour,  and  is  not  really 
satisfactory  for  use  in  concrete. 

Where  no  mechanical  means  of  washing 
the  sand  is  available,  a  tank  about  6  ft.  by 
6  ft.  by  18  in.  should  be  half-filled  with  sand 
and  a  stream  of  water  turned  in.  The  sand 


CONCRETE:    MATERIALS    AND    MIXING 


is  stirred  continuously  with  a  wooden  pole, 
the  stream  of  water  being  continued  and 
allowed  to  overflow  until  it  runs  off  perfectly 
clear.  The  remaining  water  is  then  run  off, 
and  the  sand  in  the  washer  removed. 

Circular  wash-mills,  such  as  those  used  in 
the  manufacture  of  cement,  are  not  satis- 
factory for  sand,  the  best  sand  washer  being, 
as  already  mentioned,  a  stream  of  water 
flowing  directly  forward  at  a  carefully  regu- 
lated rate,  in  combination  with  some 
mechanical  means  to  keep  the  sand  in 
vigorous  movement  whilst  it  is  immersed  in 
the  water. 

PORTLAND  CEMENT 

From  the  times  of  the  Romans  until 
Smeaton's  experiments  in  1750  there  was 
no  material  addition  to  our  knowledge  of 
mortars  and  cements.  This  intrepid  experi- 
menter, urged  by  the  necessity  of  finding  a 
reliable  mortar  for  use  in  the  construction 
of  the  celebrated  Eddystone  lighthouse, 
upset  all  traditions  by  formulating  the 
theory  that  the  hydraulic  or  setting  property 
of  a  lime-mortar  did  not  depend  on  the 
hardness  or  whiteness  of  the  stone  from  which 
the  lime  was  made,  but  on  the  presence  of  a 
quantity  of  clayey  or  argillaceous  matter 
interstratified  with  the  stone.  Aspdin  in 
1824  patented  portland  cement,  which  was 
so-called  from  a  fancied  resemblance  to  the 
well-known  Portland  building  stone.  Chiefly 
owing  to  a  lack  of  scientific  knowledge 
amongst  the  earlier  makers  of  portland 
cement,  the  first  years  of  its  history  were  full 
of  failures,  which  tended  to  bring  discredit 
on  the  industry  and  all  concerned  with  it. 

Principles  of  Cement  Manufacture. — 
An  endeavour  will  now  be  made  to  trace 
briefly  the  various  stages  in  the  experiments 
that  led  to  the  discovery  of  the  principles 
underlying  the  manufacture  of  portland 
cement.  The  practice  of  burning  limestone 
for  the  production  of  lime  to  be  used  as  a 
binding  agent  or  mortar  is  of  great  antiquity. 
The  properties  that  distinguish  the  burnt 
lime  from  its  original  form  of  linestone  are 
well  known.  Briefly,  what  occurs  is  that 
the  calcium  carbonate  (CaC08)  in  limestone, 
when  brought  to  a  sufficiently  high  temper- 
ature, loses  carbon  dioxide  (CO.,),  and  true 
lime  (CaO)  remains.  Perfectly  pure  lime 
when  treated  with  water  forms  calcium 
hydrate  (Ca(OH)2)  or  slaked  lime,  much  heat 
is  evolved,  and  the  lime  falls  to  powder. 
As  a  rule,  the  purer  the  lime  the  more 
2 


energetic  is  the  reaction,  and  conversely 
the  more  impure  it  is  the  less  intense  does 
the  action  become  and  the  less  is  the  inclina- 
tion to  fall  to  powder  evinced.  The  chief  of 
the  impurities  in  a  poor  or  slow  slaking  lime 
are  sand  or  silica  (SiO.,)  and  alumina  (Al.,03) ; 
when  combined  these  two  substances  form 
the  basis  of  all  clays  or  claylike  materials. 

Smeaton  first  pointed  out  that  the  pres- 
ence of  these  clayey  materials  renders  lime 
hydraulic  (gives  it  the  property  of  setting). 
James  Parker  was  the  first  to  use  naturally 
occurring  nodules  of  clayey  matter  which 
were  sufficiently  rich  in  lime  to  render  them 
hydraulic  when  burnt  and  ground,  and 
Aspdin  was  the  first  to  use  fairly  pure  lime- 
stone artificially  mixed  with  clay. 

The  table  shows  (roughly)  the  relative 
proportions  of  the  principal  constituents 
that  are  present  in  (a)  a  pure  or  "  fat  "  lime, 
(b)  a  lean,  partly  hydraulic  lime,  (c)  a  natural 
cement,  and  (d)  a  modern  portland  cement :- — 


Silica 
Alumina  . 
Lime 


(a) 

1 
•5 
98 


After  burning,  these  constituents  are  no 
longer  merely  mixed  together,  but  are 
chemically  combined.  That  is  to  say,  the 
lime  does  not  exist  as  such,  but  with  the 
silica  and  alumina  it  forms  compounds 
known  as  silicates  and  aluminates  and 
aluminosilicates,  just  as  lime  when  slaked 
with  water  forms  a  hydrate  ;  thus,  without 
chemical  analysis,  it  is  impossible  to  obtain 
as  such  either  the  lime  or  the  silica  and 
alumina. 

Portland  cement  may,  therefore,  be  re- 
garded as  containing  compounds  of  lime 
with  silica  or  alumina  or  both,  its  precise 
composition  depending  on  many  factors  and 
varying  slightly  with  each  sample  tested. 

Composition  of  Portland  Cement. — 
In  order  to  follow  more  clearly  the  various 
steps  in  the  manufacture  of  cement,  it  is 
desirable  that  the  principles  underlying  the 
various  processes  should  be  understood. 
Portland  cement  is  a  combination  of  three 
bodies,  and  in  order  to  bring  about  such  a 
combination  the  interacting  substances  must 
be  in  such  a  condition  that  absolutely 
intimate  union  between  the  various  particles 
may  take  place.  When  the  reacting  sub- 
stances are  liquids,  or  solids  that  are  soluble 
in  a  convenient  medium,  it  is  a  compara- 
tively easy  matter  to  bring  about  the  desired 


18 


REINFORCED    CONCRETE 


union  (and  this  also  is  the  case  when  union 
by  fusion  is  employed) ;  but  although  there 
is  no  reason  why  cement  should  not  be  made 
by  mixing  substances  in  solution  (a  method 
much  used  in  the  production  of  dental 
cements),  it  has  been  customary  hitherto 
to  heat  the  mixed  materials  to  a  state  of 
incipient  fusion  or  vitrification  in  the  com- 
mercial manufacture  of  portland  cement. 

Hence,  to  produce  a  satisfactory  portland 
cement,  the  raw  material  must  be  brought 
to  the  finest  possible  state  ;  also,  for  physical 
and  chemical  reasons,  the  finished  article 
must  consist  mainly  of  flour  or  inconceivably 
minute  particles.  Both  the  initial  and  final 
operations  in  portland  cement  making,  there- 
fore, entail  the  employment  of  methods  and 
machinery  that  are  calculated  to  bring  about 
a  state  of  intimate  union  of  the  materials 
and  uniform  fineness  of  the  product. 

The  intermediate  stage,  in  which  the  mix- 
ture is  subjected  to  a  temperature  sufficiently 
high  or  prolonged  to  bring  about  the  desired 
combinations,  is  where  the  greatest  altera- 
tions and  improvements  of  recent  years  have 
been  effected.  In  earlier  days  the  kilns, 
after  being  loaded  by  hand,  required  a  week 
or  more  of  firing,  and  then  needed  time  to 
cool ;  now,  the  mechanically  fed  and  fired 
rotary  kiln,  electrically  driven  and  con- 
trolled, turns  out  in  a  day  more  cement 
clinker  than  its  almost  obsolete  forerunners 
produced  in  a  week.  For  the  successful 
production  of  a  uniform  and  high- class 
cement,  raw  materials  of  as  regular  and 
unvarying  a  composition  as  possible  are 
essential. 

Cement  Works  in  England.— Owing 
to  the  readily  available  deposits  of  chalk 
and  clay  in  the  Thames  and  Medway  valleys, 
numerous  works  were  started  and  still  con- 
tinue in  those  neighbourhoods,  and  it  was 
thought  at  one  time  that  a  good  cement 
could  not  be  produced  save  from  these 
materials  ;  but  whilst  London  cement  still 
holds  its  own,  the  growth  of  knowledge  and 
extended  experience  has  shown  that  in 
many  parts  of  Great  and  Greater  Britain 
other  materials  exist  from  which  some  of 
the  best  cement  may  be  manufactured,  par- 
ticularly in  the  Tyne  Valley,  in  Warwick- 
shire and  Cambridgeshire.  Probably,  as 
landowners  avail  themselves  of  the  know- 
ledge and  experience  now  obtainable,  an 
increasingly  large  number  of  factories  will 
spring  up  in  the  neighbourhocd  of  the  large 
centres  of  population  where  suitable  raw 


materials  and  fuel  may  be  found,  together 
with  a  steady  demand  for  a  sound  cement. 

Preparing  Materials  for  the  Kilns. 
— Two  chief  methods  are  employed  in  pre- 
paring the  materials  for  the  kilns.  They 
are  known  as  the  "  wet "  and  "  dry  "  pro- 
cesses respectively.  The  former  is  used 
chiefly  for  chalk,  clay,  and  soft  materials, 
and  the  latter  for  argillaceous  limestones  and 
harder  rocks.  In  some  works,  a  modifica- 
tion or  combination  of  both  methods  is 
employed. 

The  Wet  Process.— This  is  used  for  most 
of  the  cement  made  in  England  at  the  pre- 
sent time,  and  is  chiefly  applied  to  the  vast 
deposits  of  chalk  found  in  Kent  and  Essex, 
in  which  districts  the  chalk  is  obtained  by 
quarrying,  and  the  men  employed  in  this 
work  are  known  as  "  chalkies."  The  material 
is  sometimes  loosened  by  blasting,  but  this 
is  a  practice  not  often  necessary.  After 
removal  of  the  bull-head  or  gravelly  soil, 
the  chalk  is  loosened  by  crowbar  or  pick 
and  slides  down  the  face  of  the  cliff,  and  is 
directed  by  temporary  wooden  shoots  into 
the  wagons  awaiting  its  descent.  Large 
quantities  of  flints  occur  in  layers  in  the 
chalk  deposits,  and  these  are  removed  either 
at  the  quarry  or  when  the  wagons  are  un- 
loaded at  the  washmill.  The  chalk  wagons 
are  conveyed  to  their  destination  by  small 
locomotives.  These  locomotives  also  bring 
the  clay  in  trucks  from  the  pit,  or,  more 
generally,  from  the  wharf  whither  it  has  been 
brought  by  barges  from  the  dredging  grounds 
on  the  Medway.  In  one  of  the  most  up-to- 
date  works  in  Great  Britain  the  clay  is  now 
dumped  into  the  washmills  at  the  pit.  The 
object  of  the  washmill  is  to  separate  the 
coarse  impurities  and  to  mix  the  fine  ones 
together  intimately. 

The  washmill  consists  essentially  of  one  or 
more  circular  tanks,  usually  about  15  ft.  in 
diameter  and  5  ft.  deep.  In  the  centre  is  a 
brickwork  pier  carrying  a  vertical  shaft  bear- 
ing radial  arms  from  which  is  suspended  a 
series  of  harrows.  The  central  shaft  is 
rotated  by  suitable  gearing,  and  the  steel 
teeth  of  the  harrows  run  in  the  annular 
space  between  the  central  pier  and  the  mill 
walls.  Into  this  mill  the  chalk  and  clay  are 
introduced  in  the  proper  proportions,  to- 
gether with  a  sufficient  amount  of  water  to 
form  with  the  disintegrated  chalk  and  clay 
a  liquid  of  creamy  consistency  known  as 
slurry.  Often  the  chalk  and  clay  are  treated 
separately  in  washmills,  and  the  mixed 


CONCRETE:    MATERIALS    AND    MIXING 


slurries  are  then  treated  in  another  mill  to 
secure  intimate  contact.  As  the  slurry 
leaves  the  mill  it  passes  through  screens  and 
fine  sieves,  so  that  any  coarse  particles  are 
retained  in  the  mill.  From  the  washmills 
the  slurry  is  led  through  the  grinding  mills, 
which  formerly  consisted  of  French  burr 
stones,  but  these  are  being  gradually  replaced 
by  tube  mills  and  more  effective  grinding 
machines. 

Both  the  materials  led  into  the  mills 
and  the  resulting  slurry  should  be  under  the 
immediate  control  of  the  works  chemist  or 
his  assistants.  The  rule-of-thumb  methods 
of  former  times  are  now  superseded  by  very 
•careful  sampling  and  speedy  analysis  which 
permit  the  composition  of  the  mixture  to 
be  accurately  adjusted,  if  necessary.  As  a 
general  rule,  the  amount  of  chalk  is  about 
three  times  that  of  clay ;  in  some  cases  the 
mater  als  are  weighed,  and  in  other  cases 
measured.  In  either  case,  the  chemist 
should  be  able  readily  to  alter  the  relative 
proportions  of  the  ingredients,  and  this  is 
generally  accomplished  by  varying  the 
amount  of  chalk,  the  clay  remaining  constant 
in  weight  or  volume.  As  the  slurry  passes 
through  the  mill  samples  are  taken  at  short, 
regular  periods  ;  these  samples  are  carefully 
mixed,  and  an  average  sample  is  then  sent 
to  the  laboratory  for  analysis.  This  analysis 
•consists  generally  of  a  speedy  and  reasonably 
accurate  estimation  of  the  calcium  carbonate 
present,  together  with  a  test  of  the  fineness 
-and  moisture  in  order  to  ascertain  whether 
the  grinding  apparatus  is  working  satisfac- 
torily. 

Formerly  the  slurry  was  pumped  from  the 
washmills  into  huge  settling  tanks  or  backs, 
where  it  was  allowed  to  dry,  and  it  was  then 
•dug  out  and  loaded  into  upright  bottle 
kilns.  After  the  slurry  had  been  pumped 
into  the  backs  there  was  little  possibility  of 
altering  its  composition,  and  so,  right  or 
wrong,  it  went  into  the  kilns.  At  a  later 
date,  the  slurry  was  pumped  from  the  wash- 
mills  into  chambers  or  tunnels  through  or 
over  which  passed  the  waste  heat  from  the 
kilns,  and  this  was  a  decided  economy, 
saving  both  time  and  labour.  But  this 
practice  is  now  giving  way  to  the  method  of 
burning  in  rotary  kilns  with  previous  mix- 
ing in  tanks,  the  slurry  being  constantly 
mechanically  stirred  in  order  to  obtain  a 
material  of  more  uniform  composition. 
This  method  is  also  of  great  advantage  to 
the  chemist,  as  it  affords  another  opportunity 


of  testing  the  composition  of  the  mixture 
and  correcting  it  if  necessary. 

The  Dry  Process. —  As  already  men- 
tioned, the  dry  process  is  often  used  for  lime- 
stones, shales,  etc.,  in  the  manufacture  of 
portland  cement,  which  cannot  be  reduced 
to  powder  or  slurry  merely  by  stirring  them 
with  water  in  a  washmill.  The  chief 
materials  for  which  it  is  used  are  the  lime- 
stones and  shales  of  the  lias  formations  in 
Warwickshire,  and  the  limestones  and  harder 
clays  of  Wales.  The  materials  are  dried 
and  reduced  to  a  fine  powder  by  means  of 
crushers  similar  to  those  used  for  grinding 
clinker.  The  powder  is  mixed  with  water 
into  a  stiff  paste  in  a  pugmill  or  mixer,  con- 
sisting of  an  open  trough  fitted  with  revolv- 
ing blades  on  a  horizontal  shaft.  The  paste 
is  broken  into  irregular  lumps  or  is  com- 
pressed into  bricks,  which  are  dried  and  then 
taken  to  the  kilns.  If  a  tube  mill  is  used  for 
crushing  the  material,  an  open  pan  mixer 
may  be  used.  Where  a  rotary  kiln  is  em- 
ployed, there  is  no  need  to  dry  the  lumps  or 
bricks  of  "  compo,"  and  some  firms  prefer 
to  use  a  very  soft  paste  and  to  load  this 
direct  into  the  rotary  kiln. 

Opinions  are  divided  as  to  the  relative 
values  of  the  cement  produced  by  the  wet 
and  dry  processes  respectively.  The  choice 
of  one  or  other  method  depends  chiefly  on 
the  nature  of  the  raw  materials,  so  that  a 
detailed  comparison  is  of  little  value,  and 
concerns  the  materials  rather  than  the 
method  employed.  For  this  reason  such 
comparisons  are  best  avoided  as  being  often 
misleading  and  based  on  insufficient  grounds. 
It  does,  however,  appear  to  be  a  fact  that 
the  clinker  produced  from  materials  treated 
by  the  dry  method  is  somewhat  denser  and 
more  difficult  to  grind  than  that  obtained 
when  the  wet  process  is  used. 

Producing  the  Cement  Clinker. — The 
mixed  materials,  in  the  form  of  liquid  slurry 
or  of  solid  lumps  or  bricks,  must  be  heated 
sufficiently  to  cause  it  to  fuse  partially  or  to 
vitrify,  in  order  to  effect  the  necessary 
chemical  combinations  and  to  produce 
cement  clinker.  Until  recently,  this  heating 
was  effected  in  kilns  of  two  types — (a)  vertical 
shaft  or  bottle  kilns,  and  (b)  continuous 
Hoffmann  kilns.  The  former  consist  of  an 
upright  shaft  into  which  fuel  and  lumps  of 
cement  mixture  are  fed  so  as  to  form  alter- 
nating layers,  in  a  manner  resembling  the 
burning  of  lime.  When  the  kiln  has  been 
filled  the  fuel  is  lighted,  and  when  it  has  all 


20 


REINFORCED    CONCRETE 


been  burned  the  clinker  is  withdrawn  from 
the  bottom  of  the  shaft.  This  intermittent 
working  produces  a  cement  of  variable 
quality,  but,  with  care,  satisfactory  results 
are  obtained.  The  Hoffmann  kiln  consists 
of  a  ring-shaped  space  in  which  the  lumps 
of  cement  mixture  are  stacked,  and  the  fuel 
is  burned  in  hollow  pillars,  about  6  ft.  apart, 
left  in  filling  the  kiln.  Its  action  is  quite 
continuous,  one  part  of  the  kiln  being  filled 
and  another  emptied  whilst  others  are  being 
burned.  Owing  to  the  regenerative  use  of 
the  air  and  fuel  gases,  this  kiln  is  highly 
economical  in  fuel,  but  the  labour  of  stack- 
ing the  lumps  of  material  and  of  withdrawing 
the  clinker  by  hand  makes  it  more  expensive 
than  the  rotary  kiln,  which  is  rapidly  re- 
placing all  other  types,  one  of  its  advantages 
being  that  it  can  burn  liquid  slurry. 

Although  rotary  kilns  were  introduced  at 
the  beginning  of  the  eighties  of  the  nine- 
teenth century,  their  extensive  and  success- 
ful use  is  a  matter  of  comparatively  recent 
history.  The  rotary  kiln  as  now  generally 
used  may  be  briefly  described  as  a  steel  or 
iron  cylinder  from  5  ft.  to  9  ft.  in  diameter 
and  from  75  ft.  to  250  ft.  in  length  ;  many 
kilns  originally  of  the  former  length  have 
been  extended,  and  now  150  ft.  is  the  more 
general  length.  This  cylinder  is  lined  with 
a  material  that  is  calculated  to  withstand 
both  the  high  temperature  and  also  the 
action  of  the  highly  basic,  almost  molten, 
clinker.  The  cylinder  is  erected  at  an  in- 
clination varying  with  the  materials  that  are 
to  be  used  and  the  intended  speed  of  rotation, 
the  modern  tendency  being  to  decrease  the 
angle  and  increase  the  speed.  The  kiln  is 
supported  on  roller  bearings  and  set  in 
brickwork  hoods  at  each  end,  several  kilns 
being  often  connected  to  one  chimney  shaft 
by  means  of  flues.  It  is  not  necessary  to 
enter  into  the  details  of  driving  and  control- 
ling the  machines  ;  electric  power  is  usually 
employed,  and  by  means  of  an  ingenious 
device  the  speed  of  rotation  and  the  fuel  or 
raw  material  feed  may  be  varied  at  will. 
Very  finely  powdered  coal,  which  has  been 
passed  through  a  rotary  drier  before  grind- 
ing, is  the  usual  fuel ;  the  coal  is  brought 
from  the  grinding  mills  by  screw  or  other 
conveyers  to  hoppers  at  the  lower  end  of 
the  kilns,  and  is  blown  into  the  latter  by 
steam  or  air  supplied  by  a  fan.  The  raw 
mixture,  or  slurry,  is  pumped  into  the  upper 
end  of  the  kilns,  down  which  it  is  carried  by 
the  rotary  motion.  During  its  passage 


through  the  kiln  the  moisture  is  first  driven 
off,  then  the  carbon  dioxide  is  removed, 
and  finally  the  lime  thus  produced  combines- 
in  the  hottest  part  of  the  kiln  with  the  silica, 
and  alumina  of  the  clay  ;  the  semi-molten 
globules  that  are  thus  formed  drop  out  at 
the  lower  end  of  the  kiln,  and  then  pas» 
through  cooling  cylinders.  Cold  air  passes- 
up  through  these  cylinders,  which  in  con- 
struction are  very  similar  to  the  kilns  ;  and 
in  this  way  the  clinker  is  cooled  and  the  now 
hot  air  is  supplied  to  the  kilns.  The  clinker 
thus  produced  is  of  a  dark  greenish  colour,  in 
pieces  about  the  size  of  hazel  nuts,  and  is- 
received  into  hopper  trucks,  which,  as  soon 
as  they  are  filled,  are  conveyed  to  the  weigh- 
ing bridge  and  thence  on  to  the  storage- 
hopper  or  dry  grinding  mill. 

Grinding  the  Clinker. — Dry  grinding 
with  burr  stones  has  now  become  quite 
obsolete,  the  work  being  done  largely  by  ball 
and  tube  mills,  pendulum  mills,  or  other 
modern  contrivances,  no  preliminary  crush- 
ing of  the  rotary  clinker  being  necessary. 

The  form  of  pendulum  mill  chiefly  used 
in  England  is  the  Griffin  mill,  consisting 
essentially  of  a  horizontal  driving  pulley, 
from  which  the  shaft  is  suspended  by  a 
universal  bearing.  To  the  lower  end  of 
the  shaft  a  crushing  roll  is  attached,  which 
is  thus  free  to  swing  in  any  direction  within 
the  pan  containing  the  ring  against  which 
the  roll  works,  the  pulverising  being  done 
between  the  roll  and  the  edge  of  the  pan. 
Just  above  the  roll  is  a  fan,  and  on  the 
under  side  are  a  number  of  ploughs.  The 
roll  is  within  the  ring,  and  centrifugal 
action  carries  it  against  the  edge  of  the 
pan.  When  the  clinker  is  fed  into  the 
mill  it  is  thrown  up  by  the  ploughs  between 
the  roll  and  the  edge  of  the  pan.  When 
ground  sufficiently  fine  the  cement  passes- 
out  through  a  screen  surrounding  the  pan, 
which  for  that  purpose  has  a  number  of 
openings  downwards  thrdugh  it.  The  whole 
grinding  part  of  the  mill  is  covered  with  a 
conical  sheet-iron  case,  and  the  revolving 
fan  draws  in  air  through  the  top  and  forces 
the  cement  out  through  the  screens. 

The  ball  mill,  briefly  described,  consists- 
of  a  cylinder  revolving  round  a  horizontal 
axle.  The  circumference  is  formed  of  per- 
forated steel  plates  arranged  to  overlap  each 
other.  Outside  the  plates  are  coarse  screens, 
beyond  which  again  finer  screens  may  be 
placed.  Sometimes  an  air-separator  replaces 
the  screens.  The  clinker  is  fed  into  the  mill 


CONCRETE:    MATERIALS    AND    MIXING 


21 


through  a  hollow  trunnion,  and  in  the  mill 
are  placed  a  number  of  steel  balls  which  are 
•carried  round  and  dropped  from  plate  to 
plate  as  the  mill  revolves,  thus  grinding  the 
material  that  is  fed  into  the  mill. 

As  a  rule,  the  ball  mill  is  only  used  for 
•coarsely  grinding  the  clinker,  which  is  then 
passed  into  a  tube  mill,  which  consists  of  an 
iron  shell  from  16  ft.  to  22  ft.  long  and 
about  5  ft.  in  diameter.  This  shell  is  about 
half-filled  with  flint  pebbles,  and  the  mill 
is  fed  and  discharged  by  means  of  hollow 
.shafts  at  each  end  ;  as  the  mill  rotates  the 
pebbles  are  carried  partly  round  up  the  side 
of  the  mill  until  a  point  is  reached  where  they 
•drop  back  in  a  cascade  upon  the  material 
below,  thereby  reducing  it,  in  time,  to  finest 
possible  powder.  The  material  to  be  ground 
•enters  at  one  end  of  the  tube  and  gradually 
works  its  way  to  the  other  end,  where  it  is 
•discharged  on  to  screens  or  into  an  air- 
separator,  the  coarse  material  being  returned 
to  the  mill  to  be  re-ground.  The  pebbles 
are  fed  into  the  mill  through  a  manhole,  and 
are  prevented  from  leaving  by  the  smallness 
of  the  apertures  in  the  mill.  The  tube  mill 
is  only  used  as  a  finishing  mill,  and  usually 
it  must  be  worked  in  conjunction  with  a 
coarse  grinding  mill,  a  ball  mill  being 
admirable  for  this  purpose. 

From  the  mills  the  now  finely  ground 
cement  is  carried  by  belt,  screw,  or  other 
conveyers  to  the  storehouse,  and  there 
packed  in  sacks  or  casks  or  stored  in  bins 
until  required. 

Regulating  the  Setting  of  Cement. — 
€ement  made  from  clinker  burnt  in  shaft  or 
Hoffmann  kilns  is  relatively  slow  in  setting, 
but  that  made  under  the  best  modern  con- 
ditions and  burnt  in  rotary  kilns  sets  with 
inconvenient  rapidity  and  requires  the  addi- 
tion of  some  retarding  agent.  Several  sub- 
stances are  available,  the  one  most  gener- 
ally employed  being  gypsum  (plaster-of- 
paris)  or  other  forms  of  calcium  sulphate 
(CaS04).  Most  specifications  of  portland 
cement  recognise  the  necessity  for  this 
retardation  of  the  setting,  and  they  permit 
the  addition  of  not  more  than  2  per  cent, 
of  one  of  the  substances  above  mentioned, 
the  addition  of  the  substance  being  made 
during  the  operation  of  grinding.  An- 
other plan  now  frequently  followed  is  the 
blowing  of  steam  into  the  tube  mill ;  in  this 
way  any  traces  of  free  lime  are  hydrated. 
The  cement  thus  manufactured  should  be  fit 
for  use  immediately  after  grinding,  which 


was  not  the  case  in  former  days ;  it  used 
to  be  inadvisable  to  use  cement  freshly 
made.  On  no  account  should  the  con- 
tractor keep  cement  long  on  hand  before 
using. 

Testing  Cement. — In  the  early  days  of 
the  cement  industry  the  methods  of  testing 
employed  were  on  all  fours  with  those 
obtaining  in  the  majority  of  the  cement 
works  of  that  period,  namely,  if  existent  at 
all,  entirely  unscientific,  rough,  and  crude. 
The  apparent  antipathy  of  the  earlier  cement 
makers  towards  chemists  or  other  trained 
scientific  workers  and  methods  was  only 
overcome  when,  as  a  result  of  numerous 
disasters,  their  aid  was  perforce  invoked. 
This  probably  accounts  for  the  many  useless 
tests  introduced  at  a  later  period  of  the 
industry,  every  engineer,  architect,  or  sur- 
veyor, in  their  anxiety  to  avoid  disaster  and 
disappointment,  devising  tests  that  pleased 
their  individual  fancy.  At  this  time,  also, 
every  works  of  note  had  its  own  system  of 
mechanical  and  physical  tests,  according  to 
the  ideas  of  those  in  control.  This  diversity 
of  opinion  and  specifications  continued  until 
the  opening  years  of  the  present  century, 
when,  in  1903,  a  Committee  on  Cement  was 
appointed  by  the  Engineering  Standards 
Committee,  which  is  supported  by  the  prin- 
cipal engineering  institutions  and  societies. 

The  sectional  committee  on  cement  which 
drafted  and  revised  what  is  now  the  recog- 
nised British  Standard  Specification  (obtain- 
able through  any  bookseller  for  5s.),  is  com- 
posed of  representatives  of  various  institu- 
tions, public  bodies,  cement  manufacturers, 
experts,  and  large  users.  Every  buyer  or 
large  user  of  cement  is  recommended  to 
obtain  a  copy  of  this  specification,  and  to  use 
only  cement  that  is  guaranteed  to  comply 
with  its  requirements.  It  will  at  once  be 
recognised  that  it  is  impossible  for  any  in- 
experienced person  to  carry  out  many  of  the 
physical  and  mechanical  tests  therein  de- 
scribed, whilst  the  question  of  ascertaining 
the  chemical  composition  is  one  for  the 
trained  analytical  chemist.  The  users  of 
cement,  therefore,  must  of  necessity  depend 
largely  upon  the  guarantee  of  the  manu- 
facturer or  employ  the  service  of  an  ex- 
perienced tester  or  expert.  If,  however,  the 
quantity  purchased,  or  the  importance  of  the 
work,  does  not  warrant  the  expense  thus 
involved,  the  user,  having  taken  the  pre- 
liminary precaution  of  obtaining  a  well- 
known  brand  of  cement,  may  for  his  own 


REINFORCED    CONCRETE 


gratification  carry  out  a  few  simple  tests 
that  will  at  once  tell  him  whether  the  cement 
is  suitable  for  the  object  in  view. 

Before  describing  these  "  unofficial  "  tests, 
it  may  be  well  if  we  briefly  enumerate  the 
requirements  of  the  British  Standard  Speci- 
fication. The  following  is  abstracted,  by 
permission  of  the  Engineering  Standards 
Committee,  from  Report  No.  12  (revised 
August,  1910),  "  British  Standard  Specifica- 
tion for  Portland  Cement,"  which  specifies 
the  limits  of  the  following  properties  :  (a) 
fineness,  (b)  specific  gravity,  (c)  chemical  com- 
position, (d)  tensile  strength  (neat  cement), 
(e)  tensile  strength  (cement  and  sand),  (f) 
setting  time,  (g)  soundness. 

For  tests  (d)  and  (e)  special  apparatus  is 
required,  whilst  (b)  and  (c)  can  only  be  pro- 
perly performed  in  a  chemical  laboratory  or 
specially  fitted  testing  department.  In  order 
properly  to  determine  (a),  a  good  balance 
and  set  of  weights  and  sieves  are  required, 
and  for  the  official  method  of  ascertaining  the 
"  setting  time  "  (/)  a  Vicat  needle  is  neces- 
sary. A  little  piece  of  apparatus  must  be 
obtained  to  carry  out  the  Le  Chatelier  test 
for  "  soundness  "  (g).  Each  one  of  these  tests 
requires  some  amount  of  experience  before 
reliable  and  concordant  results  can  be 
expected. 

It  will  at  once  be  realised  that  cement 
testing  is  not  work  that  can  rightly  be  left 
to  the  "  odd  job  "  man,  but  requires  care 
and  a  fair  amount  of  intelligence  and  common 
sense.  The  tests  formulated  above  are  such 
as  any  normal,  unadulterated,  and  well-made 
cement  will,  as  a  rule,  satisfactorily  pass. 

At  the  same  time,  it  is  possible  that 
cements  not  absolutely  in  accordance  with 
the  requirements  of  this  specification  may 
prove  satisfactory  and  reliable  in  practice, 
though  the  employment  of  cement  that  is 
guaranteed  to  pass  the  test  enumerated  is 
strongly  advocated. 

Fineness  Test  for  Cement. — The  object 
of  determining  the  degree  of  fineness  of 
cement  is  to  ascertain  that  its  particles  are 
in  such  a  condition  as  to  practically  con- 
stitute an  impalpable  powder.  It  may 
be  definitely  stated  that  it  is  only  the 
"  flour  "  in  a  cement  that  possesses  any  real 
cementitious  properties.  All  the  mcderately 
coarse  particles  are  not  only  useless,  but 
may  prove  to  be  a  source  of  real  danger. 
The  determination  of  the  fineness  is  a  test 
that  may  be  easily  carried  out  by  any 
intelligent  man.  In  order  to  make  the  test 


quantitative,  a  balance  and  weights,  and 
two  "  Standard  Wire  "  sieves  of  180  by  180, 
that  is  32,400  meshes,  and  76  by  76  (5,776) 
meshes,  per  square  inch  are  required.  One 
hundred  grams  (metric  system)  or  4  oz.  of 
cement  is  carefully  and  continuously  sifted 
for  fifteen  minutes  on  each  sieve.  The 
residue  on  the  180  sieve  must  not  exceed 
18  per  cent.,  that  is  18  grams  or  '72  oz. 
(nearly  f  oz.),  and  3  per  cent,  on  the  "  76." 
The  latter  constitutes  practically  an  unweigh- 
able  amount  except  a  delicate  balance  and 
weights  be  used. 

Quantitatively,  a  rough  test  of  the  fineness 
of  the  cement  may  be  made  by  sifting  a 
handful  of  cement  through  the  "  76  "  sieve  ; 
there  should  be  practically  nothing  left  on  the 
sieve. 

Specific  Gravity  Test. — This  takes  the 
place  of  the  now  obsolete  "  weight  per 
bushel "  test.  It  cannot  be  carried  out 
except  in  a  properly  equipped  testing-room. 
The  object  of  this  test  is  to  eliminate 
adulterated  or  lightly  burnt  cement. 

The  British  Standard  Specification  requires 
the  minimum  specific  gravity  of  portland 
cement  to  be  3-15  at  works,  and  3-10  after 
delivery  ;  taking  the  higher  figure,  1  cub.  ft. 
of  solid  cement  would  weigh  196  Ib.  It  is 
not  easy  to  get  uniform  results  in  determin- 
ing the  specific  gravity  of  a  fine  powder,  and 
the  specific  gravity  will  vary  with  the  extent 
to  which  the  cement  is  packed  into  the 
weighing  vessel,  and  also  with  the  conditions 
of  manufacture.  The  accepted  weight  of 
1  cub.  ft.  is  90  Ib.,  although  actually  it  may 
vary  between  75  Ib.  and  110  Ib.  A  cement 
ton  varies  slightly.  It  may  be  2,200  Ib., 
made  up  of  10  two-cental  sacks,  or  it  may 
be  2,244  Ib.,  consisting  of  either  11  sacks, 
each  of  204  Ib.,  or  12,  each  of  187  Ib.  All 
these  weights  are  net. 

Analytical  Test. — The  services  of  an 
experienced  analytical  chemist  should  be 
employed  if  there  is  any  doubt  as  to  the 
genuineness  of  the  cement.  The  addition 
of  ground  slag,  bricks,  or  other  adulterant 
can  only  be  proved  by  accurate  analysis. 
Certain  definite  ratios  between  the  "  basic  " 
and  "  acid  "  components  of  the  cement  are 
stated  in  the  Standard  Specification,  as  well 
as  limits  for  the  percentages  of  magnesia, 
sulphuric  anhydride,  and  water. 

Tensile  Strength  Test.— The  cement 
both  "  neat  "  and  "  with  sand,"  is  made  up 
into  briquettes  under  certain  stated  condi- 
tions, and  the  tensile  stress  of  the  briquettes 


CONCRETE:    MATERIALS    AND    MIXING 


ascertained  by  pulling  each  piece  until  it 
breaks  in  a  special  machine  after  the  bri- 
quettes have  been  stored  for  stated  periods 
of  time.  A  properly  equipped  testing-room 
and  the  services  of  an  experienced  gauger 
are  required. 

"Setting  Time"  Test.— The  study  of 
the  "  setting  time  "  of  a  sample  of  cement 
when  gauged  with  water  is,  although  appar- 
ently simple,  one  of  great  complexity.  In 
order  to  obtain  concordant  results  it  is 
necessary  to  strictly  adhere  to  the  conditions 
expressed  in  the  Specification.  A  Vicat 
needle  and  gauging  tools  are  necessary.  The 
setting  time  may  be  roughly  ascertained  by 
gauging  up  a  small  portion  of  the  cement 
with  sufficient  clean  water  to  form  a  thick 
paste.  A  pat  is  then  made  up  on  a  small 
sheet  of  glass  or  piece  of  slate,  and  the  surface 
tested  with  the  thumb-nail  until  it  is  im- 
possible to  make  any  impression  thereon. 
It  is  possible  in  this  way  to  distinguish 
between  a  "  quick "  and  "  slow  "  setting 
cement,  but  not  with  that  accuracy  obtain- 
able with  the  standard  needle.  Care  must 
be  taken  in  gauging  not  to  "  work "  the 
cement  after  it  shows  any  signs  of  "  setting." 

Soundness  Tests. — The  object  of  all  the 
suggested  tests  for  "  soundness  "  is  to  ascer- 
tain rapidly  the  probable  effect  of  time  upon 
the  set  cement.  It  is  a  practical  endeavour 
to  prove  that  the  physical  state  and  chemical 
composition  of  the  cement  are  as  nearly  as 
possible  correct.  At  present  there  are  no 
fully  satisfactory  methods  of  attaining  this 
object.  The  Le  Chatelier  test  has  been 
adopted  by  the  Engineering  Standards 
Committee  because  it  is  the  only  simple 
quantitative  method  that  has  yet  been 
devised.  Many  other  tests  have  been  sug- 
gested, but  they  are  either  not  quantitative 
or  else  require  expensive  and  complicated 
or  delicate  apparatus.  For  a  full  description 
of  the  test,  the  British  Standard  Specifica- 
tion should  be  consulted.  Briefly,  it  con- 
sists in  filling  a  small  mould,  consisting  of  a 
split  brass  ring  fitted  with  indicating  needles, 
with  the  gauged  cement.  The  mould  and 
contents  is  then  immersed  in  water  for 
twenty-four  hours,  removed,  and  the  distance 
between  the  indicating  needles  measured  in 
millimetres.  The  apparatus  is  then  placed 
in  water  again,  which  is  caused  to  boil  con- 
tinuously for  six  hours.  When  cool,  the 
distance  between  the  indicating  points  is 
again  measured,  and  any  increase  noted. 
A  figure  is  thus  obtained  which  should  not 


exceed  the  prescribed  limits  of  the  Specifica- 
tion. As  before  mentioned,  this  is  the  only 
simple  quantitative  test  for  "  soundness  " 
that  can  be  performed  without  the  aid  of 
expensive  and  delicate  apparatus. 

It  may  not  be  out  of  place,  however,  to 
briefly  describe  one  or  two  quantitative  tests 
that  require  practically  no  apparatus.  Some 
people,  before  testing  for  setting  time  and 
soundness,  spread  out  the  cement  to  be 
examined  in  a  layer  3  in.  deep  for  twenty- 
four  hours  at  a  normal  temperature;  but 
the  practice  is  not  recommended.  It 
should  be  understood  that  no  reliable  results 
can  be  expected  unless  the  operator  knows 
how  to  gauge  cement  properly  and  make  it 
up  into  a  pat  on  a  piece  of  clean  glass.  It 
is  not  a  simple  matter  to  do  this,  and  ex- 
perience is  the  only  teacher. 

An  excellent  test  is  to  plunge  a  pat,  before 
the  cement  sets,  into  cold  water.  A  good 
cement  will  easily  stand  this  test,  a  faulty  one 
will  either  fall  to  pieces  almost  immediately, 
or  show  signs  of  cracking  at  the  end  of 
twelve  or  twenty-four  hours.  A  quantitative 
"  hot "  or  accelerated  test  is  to  place  a  pat 
after  setting  into  cold  water,  which  is  then 
gently  caused  to  boil  and  kept  boiling  for 
six  hours.  The  pat  should  remain  on  the 
glass,  and  not  show  any  signs  of  cracking 
or  disintegration.  It  is  not  serious  if  the  pat 
leaves  the  glass,  but  is  otherwise  sound  and 
not  buckled  or  cracked. 

It  may  be  remarked  that  the  British 
Standard  Specification  does  not  require  a 
cement  to  undergo  or  stand  either  of  the 
tests  last  mentioned,  but  in  most  cases  any 
cement  that  complies  with  the  requirements 
of  the  Specification  will  also  pass  these 
qualitative  tests. 

NATURAL  CEMENTS 

"  Natural  "  cement  manufacture  differs 
from  that  of  portland  cement,  which  is 
always  regarded  as  an  "  artificial "  cement, 
in  one  important  particular :  for  natural 
cement,  the  material — highly  calcareous  marl 
or  septarian  nodules — is  dug,  calcined,  and 
ground,  thus  producing  a  cement  of  varying 
composition  and  quality  ;  whereas,  for  port- 
land  cement,  additions  are  made  to  the  raw 
material  in  accordance  with  the  results  of 
skilled  tests,  so  as  to  obtain  a  cement  of  per- 
fectly uniform  composition.  The  cement  is 
also  tested  at  various  points  in  the  manu- 
facturing process,  and  the  resultant  product 
is  therefore  a  reliable  and  uniform  article, 


REINFORCED    CONCRETE 


whereas  the  natural  cements  vary  greatly, 
even  at  the  same  works. 

A  natural  cement  very  inferior  to 
portland  cement  is  produced  in  Belgium, 
and  a  few  years  ago  large  quantities  of  it 
were  used  in  Great  Britain,  the  material 
being  introduced  into  this  country  by  the 
most  unscrupulous  methods.  The  packages 
were  put  up  in  close  imitation  of  those  con- 
taining genuine  portland  cement  of  British 
origin  and  often  bore  labels  of  well-known 
British  brands.  The  importation  of  this 
fictitious  portland  cement  is  now  practically 
dead,  but  as  recently  as  1904  it  is  computed 
that  nearly  234,000  tons  entered  Great 
Britain. 

SLAG  CEMENT 

Ordinary  blast-furnace  slag  without  any 
addition  does  not,  as  a  rule,  possess  cementi- 
tious  properties  ;  it  requires  the  addition  of 
lime  powder  to  make  it  hydraulic.  Cement 
made  from  blast-furnace  slag,  and  known 
as  "  slag  cement,"  has,  therefore,  chiefly 
consisted  in  the  past  of  a  mixture  of  granu- 
lated blast-furnace  slag  and  finely  powdered 
lime.  In  certain  patented  processes,  a 
cement  is  made  by  causing  the  molten  slag 
to  become  impregnated  with  alkaline  salts 
in  solution. 

A  small  proportion  of  granulated  blast- 
furnace slag  when  finely  ground  with  good 
cement  clinker  is  stated  to  make  the  con- 
crete of  which  it  forms  part  less  pervious  to 
water,  and  substantially  to  increase  its 
mechanical  strength  ;  burnt  clay,  trass,  or 
other  puzzolanic  material,  is  said  to  have 
the  same  effect.  There  are,  however,  reasons 
for  supposing  that  these  statements  have 
been  originally  made  by  those  interested  in 
the  utilisation  of  slag  cements  or  adulterated 
portland  cements,  and  that  they  are  subject 
to  the  suspicion  of  being  biassed. 

Slag  cements  are  now  made  by  a  variety 
of  processes,  and  in  some  cases  they  resem- 
ble true  portland  cements  so  closely  in  com- 
position and  properties  as  to  be  indistin- 
guishable from  them.  Most  slag  cements, 
on  the  contrary,  are  quite  distinct  from  port- 
land  cements,  and  are  nothing  more  than 
artificial  puzzolanas.  Such  cements  should 
be  rigidly  excluded  from  admixture  with 
portland  cements  or  from  sale  under  this 
title  ;  their  nature  should  be  clearly  stated, 
as  puzzolanic  cements  are  very  inferior  to 
portland  cements.  Apart  from  the  careful 
adjustment  of  the  composition  of  the  slags 


by  the  addition  of  limestone,  followed  by  a 
recalcination  of  the  mixture  in  the  kiln 
(whereby  a  true  portland  cement  is  formed),  it 
does  not  appear  probable  that  there  will  be 
any  improvement  in  the  manufacture  of 
cement  from  blast  furnace  slag  that  will 
render  it  a  keen  competitor  of  true  portland 
cement.  It  must  be  remembered  that  slag 
is  a  waste  product,  and  that  in  the  manu- 
facture of  iron  from  its  various  ores  the  com- 
position of  the  slag  must  vary  according  to 
the  character  of  the  ore  smelted,  without  any 
consideration  relative  to  its  ultimate  employ- 
ment as  a  cement. 

Iron  Portland  Cement. — For  some  years 
a  mixture  of  portland  cement  and  granulated 
blast  furnace  slag  has  been  used  in  Germany 
under  the  term  "  Iron  Portland  Cement," 
and  has  been  the  subject  of  much  discussion. 
The  portland  cement  is  made  by  mixing  the 
granulated  slag  with  lime  and  heating  to 
sintering,  and  the  clinker  so  produced  is 
mixed  with  about  three-sevenths  of  its 
weight  of  slag.  The  mixture  is  then  ground 
and  forms  the  iron  portland  cement  of  com- 
merce. A  commission  appointed  by  the 
Prussian  Minister  of  Public  Works  reported 
in  1908  that  iron  portland  cement  and  port- 
land  cement  may  be  considered  of  equal 
value  if  in  air-hardening  tests  under  standard 
conditions  the  iron  portland  cement  gives 
satisfactory  results  ;  but  this  has  not  been 
borne  out  by  the  iron  portland  cements 
placed  upon  the  British  markets,  these  having 
a  variable  composition  and  often  containing 
deleterious  substances. 

LIME  CONCRETE 

Concrete  made  with  lime  is  weaker  and 
more  porous  than  that  made  with  portland 
cement.  Lime  concrete  will  not  always  set 
properly  in  a  damp  soil,  and  it  has  therefore 
little  value  beyond  that  of  a  simple  hard  core. 
Chalk  lime  is  quite  unsuitable  for  damp 
situations,  only  the  best  hydraulic  limes 
being  permissible.  The  best  lime  that  can 
be  used  for  concrete  is  that  known  as  blue 
lias  lime,  which  is  made  from  a  limestone 
containing  approximately  79  per  cent,  of 
carbojiate  of  lime  and  17  per  cent,  of  clay  ; 
and  it  is  the  clay  content  that  makes  the 
lime  hydraulic,  that  is,  capable  of  setting 
under  water.  The  less  the  proportion  of 
uncombined  lime  present,  the  greater  is  the 
hydraulicity.  Lime  concrete  is  not  used 
for  reinforced  work  where  portland  cement 
is  obtainable. 


CONCRETE:    MATERIALS    AND    MIXING 


WATER  FOR  CONCRETE  MIXING 

The  water  used  in  mixing  the   concrete 
should  be  clean  and  preferably  fresh.     The 
use  of  hard  water  causes  a  white  efflorescence 
to  come  out  on  the  walls  after  a  time,  similar 
to  the  well-known  patches  that  appear  on 
red-brick   fronts.      On   no    account    should 
dirty   water,    or   water   containing    organic 
matter,  as  that  from  stagnant  pools,  be  used. 
Warm  water  causes  the  cement  to  set  a  little 
quicker.     An  excess  or  insufficiency  of  water 
in  mixing  is  to  be  guarded  against — 20  gals, 
of  water  per  cubic  yard  is  a  fair  amount,  i)ut 
a  larger  or  smaller  quantity  may  be  necessary, 
according  to  the  nature  of  the  aggregate. 
Whether  the  concrete  is  wet  enough  can  be 
judged  after  it  is  deposited  and  rammed  ; 
there  should  be  just  a  slight  wetness  of  the 
top    surface.     When   there    is    not    enough 
water,  the  cement  does  not  set  properly ; 
where  there  is  too  much,  the  cement  may 
be  washed  off  the  aggregate,   and,   if  the 
forms  are  roughly  made,  some  of  the  thin 
stuff  will  run  away.     Another  point  is  that 
a  moderately  wet  mixture  prevents  the  form- 
ation of  voids  and  secures  sufficient  plasticity 
to  ensure  a  complete  filling  of  the  space  round 
and  below  the  steel  reinforcement ;    but  an 
excessively  wet  concrete  contains  numerous 
globules  of  water,   which,   when  absorbed, 
leave  the  concrete  porous,  and  tend  to  accu- 
mulate on  the  surface  of  the  reinforcement, 
particularly  on  the  under  side.     According 
to  the  Proceedings  of  the  American  Society 
of  Civil  Engineers,   the  weakening  of  the 
bond  from  this  cause  was  evident  in  certain 
beams  in  which  the  adhesion  was  noticeably 
weak,  the  water  cavities  being  apparent  at  the 
bottom  and  sides  of  the  steel  bars. 

Sea-water  should  be  avoided,  as  it  con- 
tains many  impurities. 

PROPORTIONING  CONCRETE 

Correct  practice  in  proportioning  concrete 
is  based  upon  a  proper  understanding  of  the 
purpose  of  the  ingredients,  which  has 
already  been  explained.  The  cement,  in 
addition  to  its  general  binding  power,  fills 
the  interstices  in  the  sand,  and  the  sand  the 
interstices  in  the  aggregate.  Thus,  the 
coarser  the  aggregate,  the  more  the  sand 
required  ;  and  the  coarser  the  sand,  the 
greater  the  proportion  of  the  cement  that 
must  be  used,  though  the  addition  of  a  small 
amount  of  a  much  finer  sand  is  a  more 
economical  proceeding.  The  coarser  the 
cement,  the  greater  the  porosity  of  the 


resultant  concrete ;  and  by  filling  the  fine 
interstices  in  the  cement  itself  with  a  still 
finer  substance  (hydraulic  lime,  or  extremely 
finely  ground  cement-flour),  it  is  possible 
to  produce  a  non-porous  concrete. 

Round  pebbles  do  not  interlock  with  one 
another  so  well  as  do  materials  of  an  angular 
shape,  and  as  a  result  more  sand  and  cement 
are  necessary  to  fill  the  spaces  (in  concrete 
parlance  termed  "  voids  "). 

The  essential  condition  is  the  production 
of  a  solid  body  without  voids,  all  the  pieces 
of  aggregate  and  particles  of  sand  being 
united  together  by  the  cement  matrix. 

Considered  in  the  light  of  the  above, 
concrete  consists  of  but  two  component 
parts,  aggregate  and  matrix ;  and  the  sand 
may  be  regarded  as  the  finer  part  of  the 
aggregate,  its  purpose  being  merely  to 
occupy  the  spaces  between  the  coarser 
pieces.  Thus,  if  the  sand  were  omitted, 
cement  to  nearly  the  same  bulk  as  that  of 
the  sand  would  have  to  be  added. 

It  follows,  then,  that  although  success  in 
concrete  making  is  often  thought  to  depend 
entirely  upon  strict  adherence  to  a  formula, 
there  is  no  one  formula  of  any  value  for 
general  adoption ;  and  the  correct  propor- 
tions ought  always  to  be  settled  after 
experimenting  with  the  particular  coarse 
aggregate  and  sand  which  is  to  be  used 
on  the  job. 

Unfortunately  the  composition  of  concrete 
is  frequently  settled  in  a  very  haphazard 
manner.  It  is  not  uncommon  to  find 
specifications  in  which  one  part  of  cement 
is  assigned  to  so  many  parts  of  gravel,  sand, 
and  broken  stone,  without  apparently  any 
systematic  determination  as  to  whether  the 
sand  and  cement  combined  together  will 
entirely  fill  the  interstices  in  the  larger 
materials.  The  aggregate  should  by  no 
means  be  uniform  in  size,  as  already  ex- 
plained. The  average  percentage  of  voids 
in  a  well-graded  aggregate  of  crushed  stone 
is  from  36  per  cent,  to  48  per  cent. ;  in  gravel 
the  percentage  is  a  little  less,  varying  from 
30  per  cent,  to  40  per  cent.  Sand  usually 
contains  40  per  cent,  to  50  per  cent,  of 
voids — the  actual  proportion  depends  on  the 
degree  of  uniformity  in  the  size  of  the  grains. 
From  what  has  been  said,  the  reader  will 
know  better  than  to  fall  into  the  error  of 
supposing  that,  for  example,  1  measure  of 
cement,  2£  of  sand,  and  5  of  stone  will  make 
8£  measures  of  concrete.  Mixed  dry,  the 
8f  measures  would  make  theoretically  only 


26 


REINFORCED    CONCRETE 


5  measures,  or  slightly  more,  because  the 
cement  and  sand  should  just  fill  the  voids 
between  the  stones. 

Determining  Percentage  of  Voids  in 
an  Aggregate. — A  method  of  determining 
the  percentage  of  voids  in  an  aggregate 
is  by  means  of  two  watertight  vessels  of 
known  capacity  ;  for  the  larger  vessel,  if 
the  concrete  is  for  reinforced  work,  a  con- 
venient capacity  would  be  1  cubic  foot ;  and 
for  the  smaller  vessel  an  imperial  pint 
measure,  there  being  49-82  pints  in  1  cubic 
foot,  which  is  near  enough  to  take  as  50  ;  or 
the  correct  capacity  could  be  arrived  at  by 
experiment  based  on  the  fact  that  a  pint 
of  water  weighs  20  oz.  Fill  the  larger  vessel 
with  the  aggregate,  slightly  shake  down,  and 
strike  off  level  with  the  top.  Should  the 
aggregate  be  of  a  porous  material,  it  is  first 
saturated  with  water  and  drained  before 
placing  in  the  vessel,  as  otherwise  it  will 
absorb  water  into  itself  during  the  measuring 
of  the  voids,  thereby  giving  a  greater  value 
for  the  voids  than  is  actually  the  case.  Now 
pour  water  into  the  vessel  containing  the 
aggregate  by  means  of  the  pint  measure, 
until  no  more  water  can  be  introduced  ;  note 
the  amount  required,  and  take  care  to  do 
the  work  accurately  and  without  spilling 
any  of  the  water.  Frcm  the  number  of 
pints  required  to  fill  the  vessel  in  addition  to 
the  aggregate,  the  percentage  of  the  voids 
is  found  by  the  following  rule  : — 

Number    of    pints  ,T 

— — -  x     100,    or  N    X    2, 

50 

N  being  the  number  of  pints  to  fill  the  vessel 
containing  the  aggregate,  and  50  (49'82) 
the  number  of  pints  in  1  cubic  foot. 

The  following  is  a  more  accurate  method, 
and  it  involves  the  use  of  a  galvanised  iron 
bucket  and  a  weighing  machine.  First 
weigh  the  empty  bucket  denoting  weight 
by  W;  secondly,  weigh  the  bucket  full 
of  water,  denoting  weight  by  Wj  ;  thirdly, 
weigh  bucket  full  of  aggregate  or  sand, 
denoting  weight  by  W2 ;  fourthly,  weigh 
the  bucket  full  of  aggregate  and  water, 
denoting  weight  by  W$.  Then  the  cubic 
contents  C  of  the  bucket  in  cubic  feet  will  be 

W   —  W 

C  =  -  — ,  since  a  cubic  foot  of  water 

oz'4 

weighs  62-4  Ib.     The  volume  of  the  voids 

W   —  Wo 

V  =  — ^fiiTT^  •  The  percentage  of  the  voids 

Vx  100 
in  the  sand  or  aggregate  will  be  — • 


With  sand,  it  is  difficult  to  drive  out  with 
water  all  the  air  contained  in  the  voids,  and 
therefore  an  error  of  from  8  per  cent,  to  10  per 
cent,  may  be  easily  made.  This  can  be 
avoided  by  pouring  a  measured  volume  of 
dry  sand  slowly  into  a  graduated  glass  vessel 
containing  water  ;  the  sand  sinks  to  the 
bottom  free  from  air  bubbles,  and  the 
volume  of  displaced  water  may  be  measured 
and  deducted  from  the  volume  of  the  sand, 
the  difference  being  the  voids. 

As  an  illustration  of  the  first  method, 
assume  that  it  requires  24  pints  of  water  to 
fill  the  foot  cube  (50-pint)  vessel  of  aggregate ; 

24 
then  the  percentage  of  voids  will  be  r ,-  x  100 

=  48  per  cent.  By  the  second  method, 
assuming  the  bucket  to  weigh  4'12  Ib.  =  W  ; 
when  full  of  water,  41-5  Ib.  =  Wa ;  full  of 
dry  aggregate,  61  Ib.  =  Wo ;  and  when  full 
of  aggregate  and  water,  79  Ib.  =  W3. 

The    volume    C    of    the    bucket    will    be 

41-5  -  4-12 

=  (approx.)  06  cubic  toot. 


62-4 
The  volume  of  the   voids   is 


79  -  61 
"62-4 


0-29  cubic  foot. 

The  percentage  of  voids  will  be 

•29  X  100 
— =48  (approx.). 

Next  determine  the  voids  in  the  sand 
by  the  same  method  as  for  the  coarser 
aggregate.  Then  proportion  the  cement 
and  sand  so  that  the  cement  paste  will  be 
10  per  cent,  in  excess  of  the  voids. 

Now,  cement  when  mixed  with  water 
reduces  very  much  in  volume,  and  it  has 
been  found  that  it  takes  approximately 
100  Ib.  of  cement  to  make  1  cubic  foot  of 
cement  paste.  It  is  a  simple  matter  to 
provide  100  Ib.  of  cement  to  each  cubic  foot 
of  voids  in  the  sand,  allowing  an  extra  10 
per  cent,  in  excess  of  the  voids,  and  to  pro^ 
portion  the  volume  of  the  aggregate  so  that 
all  the  voids  are  filled  with  the  cement  and 
sand  mortar  with  an  excess  of  10  per  cent. 

One  bag  of  cement  (eleven  bags  to  the 
ton)  weighs  204  Ib.  net,  and  will  make 
2  cubic  feet  of  cement  paste.  This  is  a 
convenient  basis  on  which  to  proportion  the 
volume  of  sand  and  aggregate  for  the  voids 
to  be  filled  ;  the  volume  of  the  cement  being 
thus  fixed  as  one  bag,  no  measuring  or  weigh- 
ing of  cement  will  be  required,  and  the  result- 
ing concrete  will  not  be  too  large  in  volume 
to  handle  before  the  first  set  begins. 


CONCRETE:     MATERIALS    AND    MIXING 


27 


The  volume  of  sand  in  which  the  voids 
will  be  filled  with  a  given  volume  of  cement 
paste  is  found  by  the  following  rule,  which 
also  answers  for  finding  the  volume  of 
aggregate  to  be  filled  by  a  given  volume  of 

C 

cement  and  sand  :  ^—  -  —  ^-j  --  ^.  ,  where  C 

v  +  (ITT  X  V) 

is  the  volume  of  cement  paste  (or  cement 
paste  plus  sand),  and  V  the  percentage  of 
voids  in  the  sand  (or  aggregate). 

Applying  this  rule  to  a  particular  case, 
assume  that  the  voids  in  the  sand  are  38  per 
cent,  and  in  the  aggregate  48  per  cent. 
(That  is,  taking  the  bulk  of  the  sand  and 
aggregate  as  1  each,  the  voids  are  re- 
presented by  0-38  and  O48  respectively, 
these  corresponding  to  the  percentages 
named.) 

The  volume  of  sand  in  which  the  voids 
will  be  quite  filled  with  2  cubic  feet  of 
cement  paste  (made  with  one  bag  of  cement 

2 
weighing  204  Ib.  net)  will  equal  . 


0  =  4-785     cubic     feet  =  8,268    cubic 
('41o 

inches.  The  internal  dimensions  of  a  cube 
having  this  capacity  is  arrived  at  by  finding 
the  cube  root.  Thus  ^8268  =  20'22  in. 
cube,  say  20^  in.  =  the  internal  dimensions 
of  the  sides  of  the  gauge  box  for  measuring 
the  sand. 

The  volume  of  cement  and  sand  mortar 
produced  is  found  by  Vc  +  (Vs  X  (1  —  P))  : 
where  Vc  is  the  volume  of  cement  paste 
in  cubic  feet,  Vs  the  volume  of  sand  in  cubic 
feet,  and  P  the  percentage  of  voids  in  sand. 
Therefore  the  volume  of  mortar  produced 
will  be  2  +  (4-785  X  (1--38))  =  4-966 
cubic  feet. 

The  volume  of  aggregate  for  the  voids  to 
be  filled  with  this  quantity  of  cement  and 

4-966 
sand   mortar  will   be 


4-966 

•528 


•48  +  (TV  X   -48) 
=  9-405  cubic  feet,  or  9'405  X  1728  = 


16,251  cubic  inches ;  for  this  there  will  be 
required  a  gauge  box  fyl&251  =  25 -33 
inches  cube,  say,  25T5g-  in.  in  width,  depth, 
and  length,  internal  dimensions,  for  measur- 
ing the  aggregate. 

The  resulting  concrete  would  be  of 
maximum  density,  after  which  increase  of 
strength  would  be  directly  attained  by 
increasing  the  volume  of  cement  paste.  As- 
suming that  the  aggregate  had  been  more 


perfectly  graded  and  the  voids  had  been 
40  per  cent.,  the  volume  of  aggregate  re- 
quired for  one  bag  of  cement  would  then  have 
4966  4-966 

=  :  u  28 


.40  +  (Tv  x  -40) 

cubic  feet  for  the  same  volume  of  cement 
paste,  thus  increasing  the  volume  of  concrete 
1-885  cubic  feet  for  2  cubic  feet  of  cement 
paste,  and  yet  maintaining  practically  the 
same  strength. 

The  weights  of  the  bags  of  cement  should 
be  checked  to  obviate  risk  of  error. 

Specifying  Proportions.  —  In  days  gone 
by,  concrete  was  specified  as  4  to  1,  5  to  1, 
etc.,  the  sand  and  coarser  aggregate  being 
measured  together,  and  leading  to  very 
irregular  mixtures  and  results  that  could 
not  be  relied  upon.  All  experienced  archi- 
tects and  engineers  now  specify  the  sand  and 
coarser  aggregate  separately.  Thus,  a  mix- 
ture of  1  :  2  :  4  means  1  part  of  cement  to  2 
parts  sand  and  4  parts  of  coarse  material 
of  various  sizes. 

GAUGE  BOXES  AND  MEASURES 

Aggregate  Gauges.  —  Gauge  boxes  con- 
sist merely  of  four  sides  fastened  together. 
They  have  no  bottom,  so  that  when  they 
are  filled  and  levelled  off  at  the  top,  the 
gauge  can  be  lifted  off,  leaving  the  meas- 
ured materials  in  a  heap.  They  should 
not  be  made  too  high,  and  their  capacity 
should  not  be  so  great  as  to  make  them 
unwieldy.  Some  form  of  handle  should 
be  attached.  A  good  arrangement  is  shown 
at  Fig.  7,  where  the  handles  are  placed 
midway  at  the  sides.  This  arrangement 
admits  of  the  gauge  box  being  used  either 
end  up.  Projecting  handles  at  the  four 
corners  are  objectionable  as  being  in  the  way 
of  the  workmen,  and  therefore  liable  to  get 
broken  off.  The  top  and  bottom  edges 
should  have  strips  of  hoop-iron  nailed  along 
them  to  resist  wear.  It  is  convenient  to 
make  the  capacity  some  definite  measure, 
so  that  it  can  be  used  for  measuring  a  cubic 
foot  or  multiples  of  a  cubic  foot,  or  so  that  it 
can  be  used  in  connection  with  the  scientific 
system  of  proportioning  already  described. 
A  gauge  of  J  cubic  yard  capacity  is  a 
convenient  size  in  general  concrete  work, 
and  if  the  height  is  fixed  at  1  ft.,  allows  a 
fairly  large  surface  for  spreading  the  cement 
after  the  gauge  has  been  slightly  lifted  ;  the 
final  removal  of  the  gauge  is  almost  the 
equivalent  to  one  turn  over.  The  follow- 
ing example  shows  how  to  arrive  at  the 


28 


REINFORCED    CONCRETE 


dimensions  of  J  cubic  yard  gauge  1  ft.  in 
depth  : — 

i—ff-    '  =  13J   ft.  super.,  then  v/13^  = 
3  ft.  SyV  in.  nearly. 

So  that  a  gauge  3  ft.  8^  in.  by  3  ft.  8rV  in. 
by  1  ft.  deep  =  £  cubic  yard,  and  a  gauge  of 


Fig.  7. — Gauge  Box  with  Fixed  Handles 

the  same  dimensions  top  or  bottom  area,  but 
2  ft.  deep  =  1  cubic  yard.  Care  should  be 
taken  to  make  the  gauge  accurately  square, 
and  to  provide  that  it  shall  maintain  this 
form,  or  its  capacity  may  become  seriously 
diminished.  The  joints  in  the  sides  should 
be  matched  or  grooved  and  tongued  together, 
and  the  inside  should  be  rough-planed. 

Cement  Gauges. — A  wooden  gauge  for 
cement  may  resemble  Fig.  8.  It  should  be 
made  as  light  as  possible  consistently  with 
strength,  and  furnished  with  some  means  of 
transporting  it  when  charged,  the  cement 
store  being  often  unavoidably  some  distance 
from  the  mixing  platform.  As  illustrated,  the 
arrangement  for  conveyance  takes  the  form 


Fig.  8. — Cement  Gauge 

of  two  loose  carrying  bars,  fitting  under 
wrought-iron  clips  or  brackets,  fixed  to  the 
cleats  at  the  sides  with  stout  ironwood 
screws.  This  gauge  should  be  accurately 
square,  and  have  a  bottom  to  it. 

Metal  gauges  for  cement  are  lighter  and 
handier  than  wooden  ones ;  but  cement 
gauges  are  not  necessary  when  the  whole 
system  of  proportioning  is  based  on  the 
weight  of  cement  contained  in  one  bag. 
The  best  method  of  apportioning  cement  is 
by  weight,  and  not  by  cubic  measure. 


Water    Gauges. — If   the   water    is   not 
measured  accurately,  defective  concrete  will 
be  produced.    Odd  buckets  and  cans  should 
be    avoided    as    tending    to    errors,  which, 
occasionally,  become  very  serious,  and  only 
those  measuring  appliances  should  be  used 
that  have  been  carefully  examined  and  certi- 
fied by  some  responsible  person.    For  small 
quantities  of  water  a  conveniently-sized  can 
or  bucket,  as  tall  and  narrow  as  possible,  may 
be  used,  providing  that  it  has  had  its  exact 
capacity  or  some  distinguishing  mark  legibly 
painted  on  it.    For  larger  quantities,  a  cistern 
holding  just  the  amount  o£  water  required 
for  one  batch  of  concrete  should  be  placed 
above  the  mixing  board  or  machine.     This 
cistern  should  be  provided  with  an  outlet  pipe 
and  tap  as  near  to  the  bottom  as  can  be 
arranged.     A   short   piece   of  rubber   hose 
attached  to  the  outlet  pipe  is  often  a  great 
convenience.     All  buckets,  cans  or  cisterns 
should  have,  legibly  painted  inside  them,  a 
mark  to  which  they  are  to  be  filled.     This 
mark  should  be  as  near  as  possible  to  the  top 
of  the  vessel,  as  the  risk  of  adding  too  much 
water  is  thereby  reduced.     If  the  quantity 
of  concrete  is  very  large,  it  is  more  con- 
venient and  accurate  to  have  a  cistern  fitted 
with  an  overflow  pipe,  and  to  run  water 
into  the  cistern  until  it  begins  to  overflow ; 
the  disadvantage  of  this  arrangement  is  that 
it  is  troublesome  to  adjust  the  overflow  pipe 
in  the  first  instance.    Some  firms  have  found 
tanks  operated  by  siphons  to  be  convenient 
and  accurate  ;  a  cistern  controlled  by  a  ball- 
cock  is  used  to  fill  the  measuring  tank,  and 
the  latter  is  emptied  siphonically  with  such 
suddenness  that  the  amount  of  water  intro- 
duced by  the  opening  of  the  ball-cock  is  too 
small  to  cause  any  appreciable  error.   Where 
variable   quantities  of  concrete  are  to   be 
mixed,  a  cistern  with  its  capacity  at  different 
levels  painted  on  it  should  be  used.      All 
the  water  to  be  used  for  one  batch  should 
be  held  at  one  time  in  the  water  gauge. 

MIXING  CONCRETE  BY  HAND 

There  are  but  few  hard-and-fast  rules 
with  regard  to  the  mixing  of  concrete. 
Most  foremen  and  clerks  of  works  favour  a 
particular  system  of  their  own,  and  as  long 
as  the  result  in  all  cases  is  a  properly  mixed 
material  the  quickest  method  is  the  best. 
On  no  account  must  the  concrete  be  worked 
after  setting  has  begun.  In  a  method  that 
was  common  until  a  few  years  ago,  the 
materials  were  measured  out  (on  a  10  ft. 


CONCRETE:     MATERIALS    AND    MIXING 


29 


square  platform)  in  a  heap  on  the  top  of 
one  another,  the  cement  being  added  last 
from  a  wooden  box.  The  heap  was  then 
turned  over  with  shovels,  one  man  having 
an  iron  prong  ;  the  water  was  applied  gently, 
so  as  not  to  wash  the  cement  away.  The 
concrete  was  turned  often  enough  to  obtain 
a  thorough  incorporation  of  the  ingredients. 

In  another  method,  the  aggregate  was 
placed  in  a  layer  from  8  in.  to  12  in.  thick 
over  a  platform,  the  smaller  pebbles  being 
at  the  bottom.  The  cement  was  then  spread 
as  uniformly  as  possible  over  the  whole,  the 
materials  being  then  mixed  by  four  men, 
two  with  shovels  and  two  with  hoes,  the 
former  facing  each  other,  and  always  working 
from  the  outside  to  the  centre,  then  stepping 
back  and  going  over  it  again  in  the  same 
way  ;  the  operation  was  continued  until  the 
whole  mass  was  turned.  The  heap  was  then 
turned  over  again  in  an  opposite  direction, 
the  surface  of  every  pebble  thus  being 
covered  with  cement.  Two  turnings  usually 
sufficed  to  make  the  mixture  complete. 

A  method  that  once  had  the  approval 
of  engineers  is  to  spread  out  the  sand  over 
the  platform  to  the  depth  of  a  few  inches, 
and  over  this  to  spread  the  cement.  A  hole 
is  made  in  the  middle  by  means  of  a  hoe, 
and  the  water  poured  in,  the  whole  being 
then  mixed  by  means  of  shovels  and  hoes 
to  form  a  thin  paste.  The  aggregate  straight 
from  the  washing,  or  purposely  wetted,  is 
added,  and  the  whole  mixed.  This  is  a  bad 
method  ;  unless  the  mixing  board  has  a 
fillet  all  round  it,  there  is  a  risk  of  losing 
much  of  the  fine  stuff,  and  the  method  is 
slow,  giving  rise  to  the  temptation  to 
scamping. 

A  better  method  is  first  to  spread  out  the 
aggregate,  then  the  sand  over  it,  and  lastly 
the  cement  over  the  sand.  Two  shovellings 
will  then  mix  the  material  fairly  well.  It 
is  usual  to  measure  the  aggregate  in  a  gauge 
box,  smooth  out  the  aggregate  so  that  the 
sand  gauge  may  go  on  top  of  it,  and  then  fill 
the  gauge  with  sand.  In  some  cases  a  slight 
allowance  is  made  for  the  sand  which  will 
fall  into  the  voids  of  the  aggregate,  but  this 
is  scarcely  necessary  if  the  top  of  the  heap 
of  aggregate  is  smoothed  fairly  well.  Lastly, 
the  cement  is  added  from  a  wooden  or  metal 
measure,  or  possibly  straight  from  the  bag. 
Preferably,  the  water  should  be  supplied 
through  a  rose,  as  the  more  gentle  the 
application  the  better.  The  water  is  applied 
during  a  third  shovelling,  following  which 


may  be  a  fourth,  or  the  concrete  may  be 
shovelled  direct  into  the  barrows  or  other 
conveyers. 

The  Association  of  American  Portland 
Cement  Manufacturers  recommends  the 
following  method  :  the  sand  is  spread  over 
the  board  in  a  layer  3  in.  or  4  in.  thick,  and 
over  it  is  spread  the  cement.  Two  men  start 
mixing  the  sand  and  cement,  each  man 
turning  over  the  half  on  his  side,  starting  at 
his  feet  and  shovelling  away  from  him.  In 
turning  the  shovel,  the  materials  must  fall 
off  the  end  and  sides  so  that  the  materials 
are  mixed  as  they  fall.  The  mixed  sand  and 
cement  are  spread  out  carefully,  and  the 
gravel  or  stone  measuring  box  is  placed 
beside  it,  filled,  lifted  off,  and  the  gravel  is 
shovelled  on  top  of  the  sand  and  cement, 
spreading  it  evenly.  With  some  experience 
equally  good  results  can  be  obtained  by 
placing  the  gravel  measuring  box  on  top  of 
the  carefully  levelled  sand  and  cement 
mixture,  and  filling  it,  thus  placing  the  gravel 
on  top  without  an  extra  shovelling.  Add 
about  three-fourths  the  required  amount 
of  water,  using  a  bucket  and  dashing  the 
water  over  the  gravel  on  top  of  the  pile  as 
evenly  as  possible.  The  men  turn  over  the 
materials  in  much  the  same  way  as  they 
did  the  cement  and  sand,  except  that,  instead 
of  shaking  the  materials  off  the  end  of  the 
shovel,  the  whole  shovel  load  is  dumped  and 
dragged  back  towards  the  mixer,  so  that 
the  wet  gravel  picks  up  the  finer  material. 
Water  is  added  to  the  dry  spots  as  the  mixing 
proceeds  until  the  allowed  quantity  has  been 
used.  The  mass  is  turned  over  again,  and 
if  it  is  streaky  or  shows  dry  spots  it  must  be 
turned  again.  After  the  final  turning  it  is 
shovelled  into  a  compact  pile. 

"Dry"  Mixing.— The  so-called  "dry" 
method  of  mixing  is  of  interest  to  the  concrete 
block  maker  who  desires  to  remove  the 
shaped  block  from  the  block-making  machine 
with  as  little  delay  as  possible.  In  the 
United  States,  too,  it  is  used  in  cases  where 
the  contractor  is  working  on  a  time  limit, 
and  wishes  to  remove  the  centering,  etc.,  at 
the  earliest  possible  moment.  By  this 
method,  no  more  water  is  used  than  will  hold 
the  ingredients  together  when  some  of  the 
material  is  taken  in  the  hand  and  squeezed. 
Even  such  concrete  as  this  will  show  moist 
on  the  suiface  after  tamping.  The  concrete 
attains  its  strength  more  quickly  by  this 
method  than  when  it  is  made  really  wet,  but 
there  appears  to  be  no  difference  in  results  as 


REINFORCED    CONCRETE 


regards  strength  when  samples  of  the  two 
kinds  are  tested  after  an  interval  of  a  couple 
of  years. 

MACHINE    MIXING    AND    MIXERS 

The  popularity  of  reinforced  concrete  has 
been  the  underlying  cause  of  the  attention 
now  being  paid  to  the  design  and  construc- 
tion of  concrete  mixers.  The  necessity  of 
producing  a  concrete  of  high  quality,  and 
in  which  the  whole  of  the  surface  of  the 
aggregate  is  coated  with  cement,  has  had 
the  effect  of  directing  inventive  faculty 
towards  the  provision  of  a  machine  to  do 
the  work  more  efficiently  than  is  possible 
when  shovels  are  used.  A  certain  knack  is 
necessary  in  hand  mixing,  and  it  is  not  all 
labourers  that  possess  it ;  then,  too,  the 
mixing  is  heavy  work,  and  is  likely  to  be 
scamped  when  opportunities  for  doing  so 
arise.  The  introduction  of  the  mechanical 
mixer  was  the  occasion  for  much  con- 
troversy, which  has  long  since  been  settled 
in  its  favour  ;  only  now  and  then,  as  in  the 
case  of  an  important  building  at  West- 
minster erected  in  1910-12,  do  these  re- 
sponsible prefer  hand  mixing. 

In  selecting  one  of  the  forty  mixers  obtain- 
able, the  question  is  not  so  much  nowadays 
whether  it  mixes  concrete  efficiently,  but 
how  long  the  operation  takes.  To  the  time 
employed  in  the  actual  mixing  must  be 
added  that  for  charging  and  discharging, 
but  there  is  no  doubt  that  the  employment 
of  any  of  the  well-known  machines  now 
obtainable  is  sure  to  save  time  and  money. 
Machine  mixing  undoubtedly  exercises 
economy  in  the  most  costly  of  the  three 
materials — namely,  cement — inasmuch  as  a 
given  amount  can  be  incorporated  efficiently 
with  a  larger  amount  of  sand  and  aggre- 
gate than  is  possible  with  hand  mixing. 

Choosing  a  Mixer. — The  concrete  mixer 
has  to  be  designed  and  constructed  to  work 
under  very  severe  conditions,  the  mention 
of  some  of  which  will  suggest  to  the  practical 
man  the  points  to  be  borne  in  mind  when 
choosing  a  machine.  It  must  be  simple,  and 
have  as  little  mechanism  as  possible,  as  it 
will  be  tended,  not  by  mechanical  engineers, 
but  chiefly  by  labourers,  and  any  delicate 
mechanism  would  soon  be  clogged  by  the 
cement.  It  must  be  strong,  because  it  has 
to  accommcdate  heavy  batches,  and  the 
metal  in  contact  with  the  concrete  must 
be  thick  and  hard,  since  the  aggregate  has 
great  abrasive  properties.  All  parts  sub- 


ject to  wear  by  the  concrete  should  be  easily 
renewable,  and  the  makers  should  be  asked 
whether  or  not  they  can  replace  from  stock 
the  parts  in  question.  It  must  be  compact 
and  eminently  portable,  because  it  may  have 
to  be  used  on  a  number  of  floors  in  the  same 
building,  and  thence  may  require  to  be  carted 
miles  away  to  another  job.  In  its  design 
there  must  be  a  complete  avoidance  of 
arrangements  which  theoretically  are  good 
but  practically  are  bad  ;  in  other  words, 
the  machine  needs  to  have  been  designed 
by  someone  with  practical  experience  in 
mixing  concrete,  as  the  problem  is  a  different 
one  from  that  which  the  designer  of  mixing 
machines  for  soft  goods  is  required  to  solve. 
Not  only  the  actual  mixing,  but  all  the 
accessory  processes  must  be  executed 
speedily ;  it  must  be  easy  to  charge  the 
machine,  and  a  matter  of  a  few  seconds  to 
empty  it.  It  must  do  its  work  thoroughly, 
by  which  is  meant  that  the  cement  must 
be  uniformly  distributed  throughout  the 
sand  and  aggregate,  every  particle  of  which 
must  be  coated. 

Concrete  is  not  an  easy  material  to  mix, 
each  of  the  three  ingredients  presenting 
difficulties  of  its  own.  The  cement  is  liable 
to  bridge  in  a  tapered  hopper  ;  its  dust  flies 
about  and  clogs  any  mechanism  present  ;  if 
the  machine  is  not  cleaned  out  regularly  the 
cement  will  be  retained  in  angles  and 
corners  ;  and  a  moist  cement  mixture  does 
not  work  freely,  the  trouble  increasing  as 
the  proportion  of  cement  increases.  It 
might  be  thought  that  if  cement  and  sand 
were  run  into  a  revolving  drum  and  this 
speeded  up  to  a  high  velocity,  the  maximum 
of  efficiency  would  be  obtained,  but  it  is 
not  so,  as  at  a  certain  velocity  the  cement 
flies  to  the  side  and  clings  there.  Sand,  if 
at  all  damp,  refuses  to  run  unless  the  shoot 
or  hopper  is  steeply  inclined  (experience 
shows  that  the  inclination  must  be  at  least 
30  degrees  to  the  vertical).  The  difficulty 
in  dealing  with  the  aggregate  is,  first,  its 
abrasive  action,  which  rapidly  wears  out 
soft  metal ;  and,  secondly,  the  trouble  caused 
by  stones  becoming  wedged  between  two 
rotating  arms  or  between  an  arm  and  the 
side  of  the  mixing  vessel ;  the  latter  trouble 
is  not  apparent  in  most  of  the  tumbling 
barrel  mixers. 

Types  of  Mixers. — Coming  now  to 
details,  two  broad  types  of  mixer  are  at 
present  in  use,  respectively  (1)  the  batch  or 
intermittent  mixer,  which  mixes  at  one 


CONCRETE:     MATERIALS    AND    MIXING 


time  any  quantity  up  to  the  full  capacity 
of  the  mixing  vessel ;  generally  it  is  a 
rotating  drum,  to  the  inner  sides  of  which 
are  riveted  blades  which  assist  the  mixing 
action  ;  a  lesser-used  type  has  a  pan,  either 
revolving  or  stationary,  with  paddles  or 


Fig.  9. — Sectional  Diagram  showing  Principal 
of  Chicago  Cube  Mixer 

stirrers  to  agitate  the  material.  (2)  The 
continuous  mixer,  which  delivers  a  con- 
tinuous stream  of  mixed  concrete  ;  the  most 
popular  type  consists  essentially  of  an 
inclined  cylinder  with  suitable  mixing  or 
agitating  appliances. 

The  method  of  measuring  or  gauging  the 
materials  is  of  the  greatest  importance  in 
connection  with  machine  mixing.  In  the 
case  of  the  batch  mixer  the  measures  are 
boxes,  hoppers,  or  even  the  wheelbarrows 
in  which  the  material  is  conveyed  to  the 
machine  ;  but  the  most  convenient  form 
of  measurer  is  a  slap  which  may  be  lowered 
to  the  ground  to  receive  the  charge,  and 
then  in  a  few  seconds  elevated  and  tilted 
so  that  its  contents  pour  into  the  mixing 
drum. 

As  regards  continuous  mixers,  owing  to 
the  nature  of  the  mixing  process  the 
materials  have  generally  to  be  measured 
and  roughly  incorporated  in  their  dry  state 
before  they  are  introduced  into  the  machine  ; 
or,  as  an  alternative,  there  is,  as  a  part  of 
the  machine,  the  necessary  mechanism  for 
proportioning  the  ingredients.  Examples  of 
machines  so  equipped  are  illustrated  later. 

There  is  a  growing  consensus  of  opinion 
in  favour  of  the  batch  mixer  for  reinforced 
concrete  work,  while  the  other  type  is  found 
to  have  advantages  in  connection  with  large 
works,  such  as  harbours,  coast  defences,  etc., 


where  the  concrete  requires  to  be  deposited 
in  bulk,  and  where,  possibly,  immense 
quantities  are  required  in  one  place.  The 
preference  given  to  the  batch  mixer  is  due 
to  a  number  of  reasons :  (1)  It  lends  itself  to 
the  intermittent  nature  of  the  work.  (2)  It 
is  thought  that  the  clerk  of  works  can  exer- 
cise a  more  careful  supervision  over  the  pro- 
portions of  the  ingredients,  but  whether  as 
a  matter  of  fact  he  can  do  so  in  practice  is 
a  moot  point,  as  obviously  in  a  building  of 
any  size  he  has  other  things  to  do  than 
watch  a  mixer  at  work  for  several  consecu- 
tive hours.  (3)  Both  the  proportions  of 
the  ingredients  and  the  duration  of  the 
mixing  can  be  easily  varied,  whereas  it  is 
a  matter  of  difficulty  in  some  continuous 
mixers  to  make  any  appreciable  alteration 
in  the  duration  of  the  mixing  without  inter- 
fering with  the  efficiency  of  the  product,  as 
any  slowing  down  or  speeding  up  of  the 
machine  often  has  the  effect  of  necessarily 
altering  the  proportion  of  water  admitted 
to  the  mixture.  The  continuous  mixer  has 
a  different  mixing  efficiency  with  different 
classes  of  aggregate,  and,  theoretically,  with 
any  increase  in  the  proportion  of  sand  the 
inclination  of  the  cylinder  should  be  in- 
creased, this  not  often  being  possible.  (4) 
The  batch  mixer  enables  the  ingredients  to 
be  mixed  dry,  if  required. 


Fig.  10.— Chicago  Cube  Mixer  with  Engine 
and  Boiler 

The  power  for  driving  a  mixer  may  be 
either  hand  or  engine,  and  in  the  majority 
of  cases  the  larger  mixers  are  self-contained, 
a  boiler  and  engine,  an  oil  engine  or  petrol 
motor,  or  an  electric  motor  to  be  connected 
up  to  the  supply  mains,  being  mounted  on 
the  same  base  as  the  mixer.  Naturally,  the 


REINFORCED    CONCRETE 


power-driven  machine  has  a  much  greater 
capacity  than  the  hand-driven  one.  A 
warning  with  regard  to  advertised  capacities 
may  here  be  given.  Makers  often  state  most 
definitely  the  weight  or  the  number  of  cubic 
feet  of  concrete  which  their  machines  can 
mix  in  the  course  of  an  hour,  and  they 


Fig.   11. — "Cut-away"  View  of  Eclipse 
Mixing    Drum 

advertise  the  results  of  perfectly  honest 
tests  ;  but  inasmuch  as  the  capacity  of  a 
machine  must  vary  according  to  individual 
conditions — the  disposition  on  the  site  of 
the  cement,  aggregate,  etc.,  the  nature  of 
the  job,  and  the  amount  of  experience  of 
the  operatives — a  contractor  who  is  choosing 
a  machine  should  not  place  too  much  reliance 
upon  any  comparison  he  may  draw  between 
two  sets  of  figures. 

BATCH   MIXERS   DESCRIBED 

Revolving  Drum  Machines. — There  is 
any  number  up  to  a  score  of  these  from 
which  to  choose. 

Chicago  Cube  Mixer. — This  is  based  essen- 
tially on  the  old-style  tumbling  drum — a 
cubical  box  journalled  at  diagonally  opposite 
corners — but  the  shaft  in  the  old-style  mixer 
is  replaced  with  hollow  trunnions  riding  on 
rollers  and  made  large  enough  to  serve  as 
openings  for  charging  and  discharging  the 
mixer  (see  Fig.  9).  To  rotate  the  cube 
there  is  a  circumferential  rack  fastened 
around  it  on  a  drum  at  right  angles  to,  and 
midway  between,  the  trunnions  ;  this  rack, 
geared  with  a  pinion  shaft,  is  operated  by 
the  engine  shaft  in  such  a  way  that  there 
is  no  gearing  in  the  way  of  the  materials 
either  in  charging  or  discharging.  The 
sharp  corners  and  edges  of  the  cube  are 
rounded  off  to  obviate  the  possibility  of  the 


fine  material  effecting  a  lodgment.  The 
manufacturers  of  this  mixer  have  found  that 
the  batch  is  folded  over  on  itself,  and  pressed 
into  a  contracting  space  shaped  alternately 
like  a  wedge  and  like  a  pyramid  ;  and  on 
this  fact  is  based  their  statement  that  the 
mixing  is  done  by  kneading  and  not  by 
stirring.  Fig.  10  shows  the  cube  mixer, 
with  steam  engine  and  boiler  ;  another  type 
has  a  petrol  motor. 

Barker  and  Hunter. — The  special  feature  of 
this  machine  is  that  it  cannot  be  used  as  a 
continuous  mixer.  It  is  fed  while  revolving 
in  one  direction,  and  must  be  reversed  before 
the  mixture  can  be  discharged.  The  actual 
mixer  consists  of  two  truncated  cones,  whose 
bases  merge  into  a  cylinder,  the  ends  of  the 
cones  being  mounted  with  roller  path  bear- 
ings and  rotating  on  rollers  mounted  on  a 
suitable  frame.  Two  hoppers  are  fitted,  and 
the  mixer  can  be  charged  from  either  end  or 
from  both  ends,  which  are  always  open,  with 
the  cone  either  stationary  or  in  motion. 
The  makers  say  that  it  takes  twenty  seconds 
to  charge  the  material  into  the  cone,  sixty 
seconds  to  mix  them  completely,  and  ten 
seconds  to  discharge. 

Eclipse. — This  machine  is  built  shallow 
so  as  not  to  necessitate  mechanical  charging 
devices.  The  drum  has  a  shell  of  rolled 
steel  plate,  and  has  flanged  steel  heads  ;  it 
is  supported  by  trunnion  rollers,  and  is 
rotated  by  gears  meshing  into  two  gear 
rings.  At  the  charging  end  is  a  large  cir- 
cular opening  in  the  flanged  head,  and 


Fig.    12.— Eclipse  Mixer  with  Petrol^Motor 

diagonal  overlapping  charging  blades  extend 
into  the  drum.  In  the  interior  of  the  drum 
are  diagonal  blades  firmly  attached  to  the 
sides.  One  blade  extends  from  the  charging 
end  diagonally  to  the  discharging  pocket 
shown  at  the  rear  of  the  drum  in  Fig.  11. 
The  aggregates  are  carried  into  the  mix- 
ing chamber  by  conveying  blades  rigidly 


CONCRETE:    MATERIALS    AND    MIXING 


33 


attached  to  the  sides  and  ends,  so  designed 
that  the  material  cannot  slop  out  when 
mixing.  The  discharging  shoot  extends 
into  the  drum  through  the  head  at  the  dis- 
charging end.  A  shaft  extends  through  the 
drum  near  the  shoot,  and  on  it  is  mounted 
a  door  held  during  either  mixing  or  dis- 
charging by  a  strong  spring  under  tension. 
When  the  operator  throws  the  door  past  the 
centre  of  tension  in  either  direction,  the 
spring  completes  the  movement  and  holds 
the  door  until  it  is  again  held  by  the  operator. 
Fig.  12  is  a  special  design  mounted  on  a 
truck  and  equipped  with  a  petrol  motor 
housed  in  the  casing. 

Gauhe. — In  this  mixer  the  drum  (supported 
by  friction  rollers)  revolves,  while  paddles 


Fig.  13.—"  Cut-away  "  View  of  Koehring 
Mixing    Drum 

and  a  scraper  are  fixed  and  serve  to  mix 
the  material.  The  material  enters  the  drum 
through  an  opening  at  one  end,  and  the 
discharge  is  through  a  sliding  door  controlled 
by  a  lever.  The  elevating  device  is  on  the 
lines  of  the  Fawcett,  described  later,  and 
therefore,  like  that  machine,  it  requires  con- 
siderable head-room.  The  road  wheels  are 
of  large  diameter,  so  as  to  allow  trolleys  to 
pass  beneath  the  mixing  drum  to  receive  the 
concrete.  The  equipment  includes  an  auto- 
matic water-measuring  tank. 

Koehring. — In  this  the  drum  is  of  boiler- 
plate steel  with  cast  heads,  surrounding  each 
of  which  is  a  gear  ring.  The  double  drive 
has  the  advantage  that,  should  a  tooth 
break,  the  machine  can  continue  on  a  single 
drive.  An  interior  view  of  the  drum  is 
given  in  Fig.  13,  whilst  Fig.  14  is  a  photo- 
graph of  the  mixer  with  steam  boiler  and 

3 


engine,   side   loader,   and   water  measuring 
tank  (shown  above  the  mixing  drum) ;   the 


Fig.  14. — Koehring  Mixer  with  Engine 
and  Boiler 

operating  levers  and  hoisting  mechanism  are 
clearly  shown.  The  discharge  shoot  is  made 
in  two  pieces,  the  outer  end  being  stationary, 
and  the  inner  end  pivotal,  so  affording  ample 
clearance  for  wheelbarrows  in  the  discharge, 
and  permitting  the  inner  end  of  the  shoot 
to  assist  in  the  mixing  when  tilted  inwards. 
The  side  loader  is  a  short,  wide,  and  low 
bucket  with  round  corners  and  sufficiently 
low  pivotal  point  to  require  but  little  power 


Fig.   15. — Marsh-Capron  Non-tilting 
Mixing  Drum 

to  raise  it ;  it  can  be  elevated  to  almost  a 
vertical  position,  so  that  the  material  readily 
flows  into  the  drum. 


34 


REINFORCED    CONCRETE 


MarsJi-C apron. — Both    tilting    and    non- 
tilting   machines   of   this   manufacture   are 


Fig.   16. — "  Cut-away  "  View  of  Marsh-Capron 
Tilting  Mixing  Drum 

known,  and  in  each  the  drum  is  the  special 
feature.  In  the  non-tilting  style,  the  drum 
(see  Fig.  15)  is  cast  in  two  parts,  the  con- 
necting flanges  fitting  one  within  the  other 
and  centering  the  two  halves,  which  are 
securely  bolted  together.  The  steel  mixing 
blades  are  bolted  to  the  drum,  and  both 
these  and  the  steel  mixing  buckets  are  so 
placed  as  to  give  a  thorough  mixing  action 
to  the  material,  providing  an  end-to-end  as 
well  as  a  lifting  and  pouring  action.  The 
driving  gear  surrounding  the  drum  consists 
of  five  segments  absolutely  interchangeable, 
so  that  should  a  tooth  be  broken  a  segment 
can  be  renewed  at  the  labour  of  removing 
and  repl  icing  five  bolts, 
the  drum  being  kept  in 
place  'and  the  rest  of  the 
machine  remaining  undis- 
turbed. In  the  tilting 
machine  the  drum  (see 
Fig.  16)  has  its  blades 
arranged  in  serpentine 
form  and  attached  with 
brackets,  there  being 
sufficient  space  between 
blades  and  drum  to  allow 
of  thorough  washing  out. 
Fig.  17  shows  the  machine 
with  the  drum  in  the 
tilting  position. 

McKelvey. — In  this  ma- 
chine   the   drum    consists        Fig.  17. — Marsh 


of  a  short  cylinder  with  a  conical  hopper 
on  the  feed  side  and  a  trumpet-shaped 
discharge  funnel  on  the  other  side.  It  re- 
volves on  friction  rollers,  and  is  operated 
by  a  chain  and  sprocket  ring.  Its  chief 
feature  is  the  means  adopted  for  ensuring 
the  proper  mixing  of  the  material.  The 
special  device  used  (see  Fig.  18)  is  known 
as  a  "  gravity  shovel,"  and  is  pivoted  to  the 
inside  of  the  drum  ;  the  top  edge  lies  against 
the  rising  side,  collects  the  materials  in  that 
part  of  the  drum,  and,  after  it  is  filled,  the 
overflow  slides  rapidly  of?  the  lower  edge 
and  the  balance  is  carried  upward  to  a  point 
where  gravity  causes  the  shovel  to  cast  the 
material  outward  and  downward,  turning 
it  over  in  the  fall. 

Messent. — This  is  based  on  the  old  tum- 
bling barrel,  the  design  being  a  closed  vessel 
of  irregular  shape  revolving  on  an  axle. 
As  used  on  the  Dover  Harbour  works,  it 
was  mounted  on  a  steel-framed  carriage, 
there  being  one  electric  motor  to  rotate 
the  mixing  vessel  and  another  to  give  a 
travelling  motion  to  the  carriage,  the 
materials  being  thus  mixed  in  course  of 
transit  between  place  of  charging  and  the 
work. 


-Capron  Mixer,   with  Drum  in  tilting  position  " 


CONCRETE:    MATERIALS    AND    MIXING 


35 


Pioneer. — This  is  a  small  hand  mixer, 
capable  of  handling  between  250  Ib.  and 
350  Ib.  per  batch.  It  can  easily  be  worked 
by  one  man,  and  is  too  simple  to  require 
detailed  explanation,  being  simply  a  mixing 
drum  supported  on  a  horizontal  spindle. 
The  materials  are  introduced,  the  lid  closed, 


Fig.  18.— McKelvey  "Gravity  Shovel" 

the  drum  rotated  three  or  four  times,  then 
the  water  contained  in  the  tank  at  the  side 
is  allowed  to  enter  the  drum,  through  the 
perforated  shaft,  and  the  drum  is  finally 
rotated  again  a  few  times,  the  mixture  then 
being  complete. 

Ransome. — Machines  of  this  make  largely 
owe  their  conception  to  E.  L.  Eansome, 
whose  name  is  of  importance  in  the  history 
of  reinforced  concrete  in  the  United  States. 
A  variety  of  Ransome  machines  is  made, 
all  incorporating  the  vital  feature — namely, 
a  drum  with  mixing  blades  of  a  special  type. 
Figs.  19  to  24  are  photographs  of  six 
arrangements  of  the  Ransome  mixer,  there 
being  an  explanatory  inscription  to  each 
figure.  The  drum  is  not  of  the  tilting  type, 
the  discharge  being  made  possible  merely 
by  opening  a  shoot.  Each  type  of  machine 
has  a  hopper  which  measures  the  exact 
amount  of  aggregate  for  each  batch,  there 
being  in  addition  a  water  tank  which 
automatically  gauges  the  exact  amount  of 
water  required.  Within  the  drum  is  a  series 
of  steel  scoops  (see  Fig.  25)  which  plough 
through  the  material  repeatedly,  pick  it  up, 
and  carry  it  upwards  till  it  slides  out.  The 
aggregate  having  been  delivered  from  the 
hopper  into  the  drum,  the  hopper  gate  is 
closed  and  the  hopper  re-charged  in  readi- 
ness for  the  next  batch.  The  elevating 
hopper  is  so  arranged  that  it  can  be  lowered 
until  its  bottom  rests  on  the  ground.  When 
required,  a  lever  is  pushed,  and  the  elevator 
tips  the  material  into  the  hopper.  Fig.  27 


shows    two    elevations    and    a    plan    of    a 
Ransome  belt-driven  mixer. 

In  Great  Britain  the  Ransome  mixers  are 
built  by  Ransomes  and  Rapier,  Ltd.,  of 
Ipswich,  and  in  the  United  States  by  the 
Ransome  Concrete  Machinery  Co.,  Dunellin. 
The  English  and  American  machines  con- 
form to  the  same  general  patterns,  but  there 
are  minor  differences  of  detail ;  for  example, 
in  the  American  mixing  machine  the  interior 
of  the  drum  resembles  Fig.  26. 

Roll. — This  has  distinct  points  of  novelty 
(see  Fig.  28).     The  drum   consists   of  two 
cup-shaped   halves   mounted   on   a   spindle 
in  such  a  way  that  they  can  be  drawn  apart 
to  discharge  the  concrete.    The  drum  con- 
sists of  two   castings,  to   one  of  which  is 
bolted    a    sleeve    carrying    a    worm   which 
serves  to  move  one  half  of  the  drum  along 
its  spindle  when  discharging  is  necessary. 
The   mixer  is   equipped   with   an   elevator 
consisting  of   a  measuring  skip  hauled  up 
by  a  wire  rope  and  returning  by  gravity. 
Smith  Hand  Mixer. — As  shown  by  Fig. 
29,   this   machine  is   driven   by  means  of 
crank  handles — one  man  to  each  side — but 
it  is  easily  adapted  for  power  driving.    The 
ends  of  the  drum  are  sloped  inwards  until 
they  almost  meet,  forming  two  wedge-like 
chambers  united  by  a  4|-in.  slot,  extending 
diametrically  across  the  drum.     The  drum 
itself  is   suspended   on   chains,   four  guide 
rollers  being  provided  to  keep  it  in  align- 
ment and  prevent  it  from  swinging  during 
the  mixing  process.     The  principle  of  the 
mixing  can   be  understood  by  bearing  in 
mind    the    old-fashioned    hour-glass,    the 
material  from  one  chamber  having  to  pass 
through  a  restricted  opening  into  the  other 
one.     Only  two  or  three  turns  of  the  drum 
are  required.     The  charging  platform  sup- 
plied is  only  15J  in.  high,  thus  enabling  a 
wheelbarrow   to    charge    directly   into   the 
machine,  and  the  mixer  is  so  constructed 
that,  if  it  were  desired,  it  could  discharge 
directly  into  a  trench. 

Smith  Power  Mixer. — The  drum  of  this 
machine  consists  of  two  cones  united  at 
their  bases,  as  shown  in  Fig.  30.  The 
blades  which  assist  the  mixing  are  arranged 
spirally  ;  and  at  the  discharge  end  are  drip 
rings  which  prevent  the  wet  material  from 
running  down  the  cone  and  over  the  roller 
tracks.  The  drums  are  partially  lined  ;  for 
instance,  the  large  ends  of  the  cones  are 
made  of  double  thickness,  because  that  is 
where  the  principal  wear  comes.  This 


REINFORCED    CONCRETE 


lining  is  a  regular  repair  part  and  is  easily 
fitted.     With  certain  of  the  sizes  a  power 


having  a  double  conical  drum  which  is  tilted 
to  discharge  the  concrete. 


Fig. 


19. — Belt-driven  Ransome 
Mixer 


Fig.  20. — Ransome  Mixer 
with  Hoist 


Fig.  21. — Ransome   Mixer 
with  Elevating  Hopper 


Fig.  22. — Ransome  Mixer 
with  Electric  Motor 


Fig.  23. — Ransome  Mixer 
with  Boiler  and  Engine 


Fig.  24. — Ransome  Mixer 
with  Oil  Engine 


tilting  apparatus  is  supplied,  the  makers 
believing  that  the  tilted  drum  provides  the 
fastest  method  of  discharging.  It  is  only 
the  smaller  sizes  of  the  Smith  power  mixer 


Victoria. — The  drum  of  this  mixer  is  a 
cylinder  with  four  deflecting  surfaces  or 
inclined  planes  (see  Fig.  33).  Lifting 
blades  are  riveted  to  the  drum  and  extend 


Fig.  25. — Scoops  in  Ransome  Mixing  Drum 
(English) 

that  are  recommended  for  reinforced  con- 
crete work. 

Taylor. — This    resembles    the    Smith    in 


Fig.  26. — Scoops  in  Ransome  Mixing  Drum 
(American) 

from  the  discharge  end  well  towards  the 
feed  end.  As  the  drum  rotates,  the  lifting 
blades  elevate  the  material,  which  drops 


CONCRETE:    MATERIALS    AND    MIXING 


37 


upon  the  inclined  planes  and  is  thus  thrown 
across  the  drum  and  returned  again  by  the 
opposing  deflectors.  The  process  is  repeated 


the  spout  is  inserted  into  the  drum  the  mass 
falls  from  the  blades  upon  it,  and  the  con- 
crete flows  into  the  receptacle  prepared  for 


twice  for  each  revolution,  and  submits  the 
mass  to  twelve  distinct  mixing  actions  per 
turn  of  the  drum.  The  complete  machine, 
on  a  truck,  with  engine  and  boiler,  side 
loader,  and  automatic  water  tank,  is  shown 
in  Fig.  31.  There  is  a  low  feed  level,  which 
is  of  especial  advantage  in  a  portable  plant 
charged  by  wheelbarrows.  For  the  purpose 
of  emptying  the  mixture,  a  swinging  spout, 
pivoted  below  the  discharge  opening,  is  pro- 
jected into  the  drum.  It  will  be  understood, 
on  referring  to  Fig.  31,  that  the  spout  is 
pivoted  to  the  vertical  frame  shown,  and 
easily  swings  forwards  or  backwards.  When 


c  _  _ 


Fig.  27, — Elevations  and  Plan  of  Ransome 
Belt-driven  Mixer 


Fig.  28.— Roll  Mixer,  with  Elevator  raised 


REINFORCED    CONCRETE 


it.  The  flow  is  under  easy  control,  as  the 
shoot  may  be  removed  from  the  drum  at 
any  moment  by  a  mere  turn  of  the  wrist. 
Any  quantity,  from  a  quart  to  the  full  mixing 
capacity,  can  be  withdrawn.  The  charging 
skip  is  shown  in  the  elevated  position  in 
Fig.  31,  and  in  its  bottom  position  in  Fig. 
32.  This  skip  will  hold  a  complete  batch, 
and,  as  it  progresses  upwards  on  the  curved 
guides  shown,  its  nose  is  thrust  into  the 
feed  opening  of  the  drum.  Care  has  been 


of  concrete  ;  for  this  purpose  the  gauge  is 
set  for  the  maximum  capacity,  the  tank  is 
allowed  to  fill,  and  the  valve  is  then  opened. 
The  pointer  on  the  outside  will  then  fall,  and 
a  glance  at  the  gauge  will  show  at  any  instant 
how  much  water  has  run  out.  The  necessary 
water  for  a  batch  in  a  large-size  mixer  is 
best  obtained  by  measuring  the  water  into 
the  mixing  drum  in  two  lots  ;  this  is  safer, 
as  a  rule,  than  introducing  all  the  water  at 
one  time.  When  this  automatic  water  tank 


Fig.  29.— Smith  Hand-driven  Mixer 


taken  to  make  the  feed  angle  steep  to  pre- 
vent clogging,  and  to  increase  the  rapidity 
of  the  feed.  The  makers  provide  extensions 
for  the  loader  frame  when  required,  so  that 
the  skip  can  be  lowered  to  receive  the  batch 
on  a  level  below  the  mixer.  In  the  two  com- 
plete views  a  rectangular  tank  will  be  noticed 
at  the  top.  This  is  an  automatic  water 
tank  of  18-gal.  capacity,  with  a  gauge  which 
can  be  set  so  that  the  tank  will  supply 
uniform  quantities  of  water  varying  from  a 
few  drops  up  to  the  full  capacity  of  18  gal. 
The  tank  serves  as  an  indicator  to  show  the 
amount  of  water  introduced  into  any  batch 


is  a  part  of  the  equipment,  it  is  necessary 
to  supply  the  water  to  it  under  pressure,  or 
from  a  reservoir  placed  at  a  higher  level 
than  the  tank.  The  makers  fit  either  steam 
engine,  petrol  engine,  or  electric  motor  to 
their  mixers,  and  they  particularly  recom- 
mend the  slow-speed  electric  motor  which 
may  be  geared  direct  to  the  mixer. 

Other  Batch  Mixers. — The  drum  mixer 
is  not  the  only  type  of  batch  machine.  There 
are  a  few  examples  of  mixers  having  pans 
and  stirrers. 

Express. — This  has  a  stationary  pan  to 
hold  the  material  (see  Fig.  34),  revolving 


CONCRETE:    MATERIALS    AND    MIXING 


39 


in  it  being  a  series  of  paddles  and  rakes 
attached  to  arms  which  project  from  a  central 
power-driven  capstan  head.  By  means  of 
two  discharge  doors  in  the  bottom  of  the 
pan  the  smaller  size  mixer  can  be  emptied 
in  fifteen  seconds,  both  feeding  and  dis- 
charging taking  place  while  running. 

Faivcett. — The  material  is  contained  in  a 
fixed  drum  in  which  a  number  of  arms  or 


filled,  ascends  along  a  pair  of  inclined  guides 
by  means  of  a  wire  rope  which  is  wound  on 
a  drum  actuated  through  a  friction  wheel. 
When  the  skip  arrives  at  the  top,  it  tips 
automatically  into  the  mixing  vessel  and  is 
then  lowered  for  another  charge. 

Gaspary. — The  hand-driven  mixer  (Fig.  36) 
has  a  trough  capable  of  treating  up  to  45 
cub.  ft.  an  hour,  according  to  size.  Above 


Fig.  30. — Smith  Power-driven  Mixer 


paddles  rotate  in  different  directions,  the 
arrangement  being  such  that  when  the  dis- 
charge door  at  the  bottom  of  the  drum  is 
opened,  the  revolving  paddles  scrape  the 
concrete  out  (see  Fig.  35).  The  mixing  is  of 
an  efficient  character,  and  the  open  pan 
enables  the  attendant  to  see  when  the  con- 
crete has  attained  an  even  colour  through- 
out. The  machine  has  an  elevator  for 
measuring  and  feeding  the  charge,  and  an 
automatic  water  tank  for  measuring  the 
water.  The  elevator  is  a  skip  which,  when 


the  tilting  trough  is  a  small  water  tank,  the 
water  reaching  the  materials  through  a  per- 
forated pipe.  In  the  power-driven  mixing 
machine,  there  is  a  drum  with  blades 
which  rotate  in  either  direction,  this  change 
in  the  direction  being  of  advantage  for 
cleaning  and  emptying  the  mixing  drum, 
and  allowing  of  any  stones  stuck  fast  be- 
tween the  drum  and  the  blades  being  in- 
stantly removed.  Continuous  machines  are 
also  made  by  the  Gaspary  firm,  as  noted 
on  a  later  page. 


4o 


REINFORCED    CONCRETE 


Fig.  32. — Victoria  Mixer,  with  Skip  Lowered 


Oehler. — This  is  a  Swiss  machine  - 
resembling  the  Fawcett,  but  the 
paddles  rotate  in  one  direction  only. 
Open  -  Drum. — This  mixer  has 
a  cylindrical  drum,  normally  up- 
right, with  conical  top  which 
facilitates  discharging.  The  sides 
of  the  drum  are  of  steel  plate, 
but  the  bottom  is  a  casting  having 
a  circumferential  rack,  by  means 
of  which  the  drum  is  rotated 
through  gearing.  The  drum  is 
carried  by  a  supporting  frame,  in 
which  is  a  ball-bearing  for  a 
short  vertical  shaft  which  pro- 
jects into  the  mixing  drum.  The 
mixing  is  efficient,  and  is  much 
facilitated  by  parallel  blades 
riveted  to  the  sides  of  the.  drum. 
The  drum,  being  so  near  the  ground, 
can  easily  be  charged  from  wheel- 
barrows on  a  loading  platform, 
while  for  discharging  there  is  a 

hand  or  power  arrangement  for  tilting  the  Pansy. — As  shown  in  Fig.  37,  there  is 
drum.  The  capacity  varies  from  2J  to  18  an  annular  vessel  which  runs  on  a  ball- 
cub,  ft.,  according  to  the  size  and  style.  bearing  cast-steel  base,  and  is  driven  through 

toothed  gearing.  Through  the 
centre  of  the  machine  is  a  per- 
pendicular shaft,  to  which  are 
hung  angle-iron  frames,  and  to 
these  again  adjustable  ploughs 
are  fixed.  The  pan  revolves  at 
the  rate  of  ten  or  twelve  revolu- 
tions per  minute,  and  as  the 
ploughs  remain  stationary,  the 
materials  are  thoroughly  mixed 
in  a  short  time.  The  materials 
are  gauged  in  the  ordinary  way 
on  a  platform  above  the  ma- 
chine, and  tipped  directly  into 
it,  being  there  levelled  by  a 
scraper,  the  ploughs  lowered  and 
the  material  mixed  dry.  Next, 
water  is  added,  and  the  materials 
further  mixed.  When  mixing 
is  complete,  the  ploughs  are 
raised,  the  scraper  lowered,  and, 
by  means  of  the  trapdoors  in 
the  bottom,  the  mixed  concrete 
passes  into  a  receptacle  prepared 
for  it.  The  doors  close  auto- 
matically by  means  of  a  special 
catch,  and  as  soon  as  the 
material  is  all  discharged  the 
pan  is  ready  for  another  batch. 
Whalley. — In  this  machine  the 
Fig.  31.-Victoria  Mixer,  with  Skip  Elevated  materials  are  discharged  from  a 


CONCRETE:    MATERIALS    AND    MIXING 


hopper  (see  Fig.  38)  into  a  stationary  pan 
and  the  mixing  is  done  by  stirrers 
mounted  on  arms  projecting  from  a  verti- 


for  hand  use,  and  which  may  be  either 
movable  or  stationary  according  to  require- 
ments. The  materials  have  to  be  fed  in 


Fig.  33. — "  Gut-away  "  View  of  Victoria 
Mixing   Drum 

cal  shaft.  Each  stirrer  comprises  a  vertical 
cutting  part  and  a  scraping  part,  the  latter 
being  almost  in  contact  with  the  bottom 
of  the  pan.  The  stirrers 
tend  to  force  the  mate- 
rial towards  the  sides, 
and  when  the  mixing 
has  been  completed  a 
door  is  opened  and  the 
mixture  discharged  sim- 
ply by  the  continued 
rotation.  The  parts  of 
the  machine  liable  to 
wear  or  replacement  are 
the  bottom  portions  of 
the  stirrers,  and  they  can 
be  readily  renewed. 

CONTINUOUS  MIXERS 

N  o  n  -  p  r  o  portioning 
Machines. — These  are 
of  much  simpler  con- 
struction than  the  mixers 
fitted  with  proportioning 
or  measuring  arrange- 
ments, and  a  few  exam- 
ples will  now  be  briefly 
described. 

Gaspary.  —  Only  the 
smallest  of  the  Gaspary 
continuous  mixers  need 
be  mentioned  here.  Fig. 
39  shows  a  power-driven 
machine,  which,  how- 
ever, can  be  adapted 


Fig.  34. — Express  Mixer 

measured  quantities,  the  water  being 
obtained  from  a  supply  tank  shown  above 
the  mixing  cylinder,  there  being  a  perfor- 
ated pipe  extending  from  the  tank  right  into 
the  cylinder.  The  dry  mixture  enters  the 
cylinder,  is  wetted  and  mixed,  the  mixing 
action  being  assisted  by  shovels  which  are 
turned  by  the  rotation  of  the  cylinder. 

Mason. — The  feature  of  this  machine  is 
a  cylinder  slightly  larger  in  diameter  at  one 


Fig.  35. — Fawcett  Mixer 


REINFORCED    CONCRETE 


end  (the  discharge  end)  than  at  the  other 
where  the  feed  hopper  is  situated  (see 
Fig.  40).  The  cylinder  is  mounted  on  a 


Fig.  36. — Gaspary  Hand-driven   Tilting   Trough 
Mixer 

central  spindle  and  has  ribs  to  assist  the 
mixing.  Machines  for  hand  or  power  are 
made. 

Gravity  Mixers. — No  power  is  required  in 
the  operation  of  a  gravity  mixer,  the  force 
of  gravity  alone  being  relied  upon  to  do  the 
work.  The  materials  are  shovelled  into  the 


number  of  baffle  plates.  The  rows  of  pins 
are  staggered  with  respect  to  the  rows 
immediately  above  and  below  them,  and 
it  follows  that  when  the  materials  fall  from 
the  hopper  on  and  off  the  pins  and  deflectors, 
they  become  incorporated.  The  water  is 
added  (by  means  of  a  flexible  hose)  half-way 
down,  this  giving  the  materials  a  chance  to 
get  partly  mixed  before  being  wetted. 

The  Owens  is  a  gravity  mixer  in  which, 
there  is  a  steel  shoot  about  7  ft.  long  con- 
taining three  sinuous  mild  steel  bars  extend- 
ing in  one  length  from  the  top  to  the  bottom. 
The  bars  are  fixed  rigidly  at  the  top,  but  at 
the  bottom  are  held  loosely  by  means  of  a 
|-in.  pin,  which  passes  through  them.  In 
addition,  there  are  eight  large  and  six  small 
baffle  plates  fixed  to  the  sides  of  the  shoot. 
To  facilitate  cleaning,  there  is  an  inspection 
door.  The  water  supply  pipe  is  fixed  behind 
the  lower  edge  of  the  top  baffle  plate,  and 
is  perforated  both  back  and  front,  the 
whole  of  the  baffles  and  bars  receiving  a 
spray  of  water.  The  materials,  having  been 
introduced  in  any  convenient  manner,  fall 
through  the  shoot  and  are  alternately  split 
up  into  two  columns  and  then  united, 
this  action  being  repeated  eight  times. 

Proportioning  or  Measuring  Machines. 
— Brief  particulars  will  now  be  given  of 
eight  examples  of  continuous  mixers  with 
provision  for  proportioning  the  concrete 
ingredients. 

Bolte. — This  is  an  American  machine  (see 
Fig.  41)  adapted  for  hand  or  power,  accord- 


Fig.  37. — Pansy  Mixer 


top  end  of  a  vertical  or  inclined  shoot,  a 
common  pattern  of  which  has  a  large  num- 
ber of  round  pins  penetrating  the  shoot  from 
front  to  back,  there  being  in  addition  a 


ing  to  size.  The  principle  of  this  machine 
is  the  conveying  of  proportioned  quantities 
of  cement  and  aggregate,  by  means  of  a 
rubber  belt,  into  a  mixing  cylinder,  where 


CONCRETE:    MATERIALS    AND    MIXING  43 

the  material  is  agitated  by  six  blades.  The  may  be  taken  as  representative.  It  has 
hoppers  are  shown  in  Fig.  41a.  The  belt  three  hoppers  or  pockets  for  automatic- 
forms  the  bottom  to  them  and  draws  off  ally  proportioning  aggregates,  sand,  and 

cement,  and  the  proportioning 
device  works  without  the  use  of 
gears,  sprockets,  or  chains.  The 
pockets  .on  each  side  are  oper- 
ated by  one  sliding  belt,  which 
moves  forward  and  backward  on 
rolls  and  acts  as  a  plunger  for 
delivering  the  material.  The 
sliding  part  is  moved  by  two 
crank  arms  attached  to  the  square 
shaft,  which  also  passes  through 
the  cement  hopper,  where  its 
reciprocating  action  prevents  any 
bridging  of  the  cement.  Each 
material  can  be  proportioned  to 
a  nicety,  and  the  materials  can 
be  fed  in  from  one  or  both  sides. 
The  mixing  device  is  a  special 
feature,  and  unlike  any  other 
mechanism  described  in  these 
pages.  It  consists  of  seven  steel 
blades  twisted  spirally  and  bolted 


Fig.   38. — Whalley  Mixer,  with  Engine  and  Boiler 


at  each  end  to  a  solid  casting, 
as  shown  in  Fig.  43.  This  con- 
struction cuts  through,  lifts,  and 

the  materials  through  adjustable  gates,  pours  the  material,  the  result  being  a 
discharging  them  into  the  cylinder,  where,  most  efficient  mixing.  Between  the  engine 
15  in.  from  the  lower  end,  the  water  is  intro- 
duced by  means  of  a  perforated  pipe  con- 
nected to  the  water  tank  above.  As  it  is 
well  known  that  cement  and  sand  do 
not  readily  flow  at 
all  times,  the  hop- 
pers are  fitted  with 
agitators,  the  cement 
agitator  being 
worked  by  a  cam 
on  the  rear  belt 
shaft  and  the  sand 
agitator  consisting 
of  a  worm  on  a  con- 
tinuation of  the 
cylinder  shaft,  this 
being  clearly  shown 
in  Fig.  41a.  These  * 
mixers  are  essen-  ; 
tially  portable,  and  V, 
can  be  taken  from 
floor  to  floor  as  the 
work  proceeds. 

Coltrin. — Machines  of  this  make  are  made  and  the  mixer  is  a  friction  clutch,  which, 
in  eight  sizes,  between  which  there  are  besides  being  generally  useful,  has  one 
various  differences  of  detail,  but  for  the  pre-  special  advantage  :  should  a  large  stone 
sent  purpose  the  No.  14  machine  (Fig.  42)  get  into  the  mixer  accidentally  and  bind 


Fig.  39. — Gaspary  Drum-type  Continuous  Mixer 


44 


REINFORCED    CONCRETE 


between  the  blades  and  the  vessel,  the 
clutch  will  slip,  so  avoiding  breakage  of  the 
mixer.  The  water  tank  with  special  pro- 


Fig.   40. — Mason  Mixer 

portioning  arrangement  is  furnished  with  all 
machines  for  use  where  regular  direct  pres- 
sure is  not  available. 

Carey-Latham. — In  this  machine,  chain 
and  bucket  elevators  are  employed,  one  for 
the  sand  and  another  for  the  aggregate,  to 
feed  the  mixing  cylinder,  which  is  inclined 
towards  the  discharge  end  at  an  angle  of 
8  degrees,  revolves  on  friction  rollers,  and 
has  projections  on  its  inner  surface  to  assist 
the  mixing.  The  cement  is  fed  to  the 
cylinder  by  means  of  a  worm.  Other  types 
of  machine  bearing  this  name  have  been 
used,  but  the  above  is  the  most  generally 
useful. 

Gaspary. — The  funnel-dish  mixer  made 
by  this  firm  has  a  number  of  funnel-shaped 
reservoirs  open  at  the  bottom,  below  them 
being  rotating  blades.  The  materials  are  fed 
into  the  reservoirs,  passed  through  the 
adjustable  openings,  dropped  upon  the  pro- 
jecting rims  of  the  blades,  and  are  swept 
down  into  a  mixing  trough,  in  which  a  screw 
is  working  and  in  which  the  moistening  is 
effected. 


of  bridging  of  the  sand  and  other  materials,, 
without  employing  agitators.  The  appear- 
ance of  a  typical  machine  is  shown  in 
Fig.  4i.  The  three  hoppers  with  their  feed 
mechanism  deliver  the  materials  to  the 
mixing  trough  at  the  rate  of  from  35  to 
40  discharges  per  minute  for  each  material. 
The  sand  and  cement  drop  in  the  same 
place,  and  the  dry  mixing  begins  imme- 
diately and  continues  for  more  than  half 
the  entire  length  of  the  trough.  Two  water 
sprays  are  provided,  one  spraying  directly 
into  the  trough,  and  the  other  into  the 
aggregate  before  it  is  discharged  into  the 
dry  mixture  of  sand  and  cement,  the  object 
being  to  cause  the  fine  stuff  to  adhere  to 
the  aggregate  instantly.  Each  spray  has  a 
separate  regulating  valve.  Passing  through 
the  cylinder  is  a  square  shaft  to  which 
numerous  paddles  are  bolted. 

Nims. — In  this  machine,  the  mixing  vessel 
is  of  cast  iron  in  one  piece,  shaped  some- 
thing like  a  pair  of  cubes  interpenetrating 
diagonally  and  revolving  on  the  long  axis. 
A  belt  delivers  the  materials,  which  have  been 
proportioned  in  a  special  form  of  measurer, 
to  the  mixer.  The  mechanism  for  measuring 
consists  of  a  wheel  15  in.  in  diameter  and 
11  ft.  long,  revolving  under  a  hopper  which 
is  12  ft.  long.  In  the  wheel  are  receptacles 
which  receive  the  materials  as  they  fall  from 
the  hopper,  and,  as  the  wheel  continues  to 
rotate,  deliver  them  to  the  belt  conveyer 
below.  In  the  hopper  are  partitions  which 
can  be  moved  to  vary  the  proportions.  As 
above  described,  the  mixer  acts  on  the  con- 
tinuous principle,  but  by  fitting  an  auto- 


GATE  FOR 

AGGREGATE 


Fig.  41.— Bolte  Mixer 

Kent. — A  variety  of  machines  are  made 
by  this  firm,  the  principal  being  a  three- 
hopper  machine  in  which  very  carefully 
designed  arrangements  are  employed  to  pro- 
portion the  materials  accurately.  Special 
devices  have  for  their  purpose  the  prevention 


Fig.  41a. — Hoppers  of  Bolte  Mixer 

matic  gate  to  the  discharge  end,  it  can  be 
used  as  a  batch  mixer. 

Perfect. — As  shown  by  Fig.  45,  there  are 
in  this  mixer  three  distinct  hoppers,  in  con- 
nection with  each  of  these  being  a  revolving 
feed  drum  with  deep  flanges  at  each  end, 


CONCRETE:    MATERIALS    AND    MIXING 


and  a  regulating  gate  for  proportioning  the 
material  set  by  a  lever  moving  over  a  gradu- 
ated quadrant.  The  feed  drums  make  only 
one  or  two  revolutions  a  minute,  and  a 
regulator  strikes  of?  the  cement  and  allows 
a  fixed  quantity  to  pass  as  a  layer  of  uniform 
width  and  thickness.  The  stream  of  cement 
combines  with  a  stream  of 
sand,  and  then  falls  on  a 
stream  of  aggregate,  passing  cc 
thence  to  the  mixing  cylin- 
der, in  which  is  an  adjust- 
able water  spray. 

Trump. — This  type  of 
machine  requires  but  little 
power,  as  gravity  to  some 
slight  extent  is  utilised. 
The  measuring  device  consists  of  three 
cylinders  for  the  sand,  aggregate,  and 


45 

the  knife  can  be  altered  to  vary  the  pro- 
portion. The  principle  of  the  measuring 
device  is  clearly  illustrated  by  Fig.  46. 


Fig.   43.—  Coltrin  Mixing  Blades 

The   materials,    having   been   proportioned, 
flow  together  in  the   form   of  one  stream 


Fig.  42.—  Coltrin  Mixer 


cement    respectively,    and    from    this    the 
materials    pass    to    three    rotating    tables, 


Fig.  44.— Kent  Mixer 

where  they  meet  fixed  knife  edges  that 
scrape  the  materials  off  into  a  trough 
below  in  required  proportions.  The  set  of 


Fig.  45.— Perfect  Mixer 

(see  Fig.  46),  which  falls  into  a  cylinder, 
where  the  materials  are  moistened  by 
water  sprayed  from  a  perforated  pipe.  In 
the  cylinder,  steel  paddles  rotate  on  a  shaft 
and  complete  the  mixing. 

TESTING  EFFICIENCY  OF  MIXING 

Of  the  many  methods  that  have  been 
proposed  for  testing  whether  concrete  has 
been  properly  mixed,  the  most  practical 
and  the  one  best  suited  for  adoption  in 
everyday  work  is  that  due  to  Dr.  J.  S.  Owens, 
and  described  by  him  before  the  Society  of 
Engineers.  It  is  based  on  the  principle  of 
sampling  a  heap  of  concrete  at  different 
places,  and  then  seeing  whether  all  the 


REINFORCED    CONCRETE 


samples  contain  the  same  relative  propor- 
tions of  stone,  sand,  and  cement.  Each 
sample  is  put  into  a  tall  glass  cylinder 
nearly  filled  with  water,  shaken  up,  and 
allowed  to  settle.  The  rate  of  settlement 
of  cement  in  water  is  about  thirty  times 
as  slow  as  that  of  the  sand,  which,  in  turn, 
is  slower  than  that  of  the  larger  particles  of 
stone  ;  as  a  result  of  the  variable  rates  of 
deposition  the  stones,  sand  and  cement 
settle  in  distinct  layers,  whose  depth  is  pro- 
portional to  the  amount  of  the  material 
present  in  the  sample.  Thus  it  is  possible 
to  tell,  with  a  fair  degree  of  accuracy, 
whether  each  sample  was  compounded  with 
the  same  proportions  of  cement,  sand,  and 
coarse  aggregate ;  obviously,  the  test  re- 
quires reasonable  care  in  its  execution. 


barrows  of  the  usual   type,    or  they  may 
have  an  iron  framework  supporting  a  steel 


Fig.  47. — Ransome  Concrete  Cart 


CONVEYING   CONCRETE 

It  always  happens  that  either  the 
raw  materials  have  to  be  conveyed 
from  their  storage  place  to  the  stage 
or  mixer  or  that  the  concrete  is  made 
close  to  where  the  materials  are  stored 
and  then  has  to  be  carried  to  the 
place  where  it  is  to  be  used.  The 
means  of  conveyance  employed  em- 
brace barrows,  handcarts,  derrick 
hoists,  bucket  elevators,  and  special 
hoisting  systems,  such  as  the  Ransome, 
that  have  been  particularly  designed 
for  use  in  connection  with  concrete 
buildings. 

The  barrows  may  be  deep  wooden 


Fig.  46.— 
"  Cut-away  " 
View  show- 
ing Principle 
of  Trump 
Mixer 


box  in  which  the  concrete  is 
carried,  the  design  being  such 
that  the  load  is  carried  as  much 
over  the  wheel  as  possible. 
A  convenient  type  of  iron 
barrow  is  arranged  in  such  a 
way  that  the  box  is  pivoted  on 
the  axle  at  each  side  of  the 
wheel,  so  that  it  can  be  tilted 
forward  to  discharge  its  load.  In 


Fig.  48. — Ransome  Concrete  Skip  or  Bucket 


Fig.  49. — Ground  Plan,  Top  Plan,  and  Two  Elevations  of  Ransome  Tower 


48 


REINFORCED    CONCRETE 


engineering  work,  but  not  often  in  general 
building  work,  it  is  economical  to  have  bar- 
rows or  tray  dumpers  running  on  fixed  rails  ; 
the  boxes  need  to  be  made  of  substantial  steel 
plate  strengthened  round  the  top  edge  to 
avoid  bending  and  buckling,  and  there  must 
be  an  arrangement  for  tilting  the  buckets. 
One  of  the  best  forms  of  hand  conveyers  is 
the  Ransome  cart  (Fig.  47),  which  has  a  work- 
ing capacity  of  from  5  to  5J  cub.  ft.  of  wet 
concrete.  The  wheels  are  42  in.  in  diameter 
with  eighteen  staggered  spokes,  the  axle, 
1 J  in.  in  diameter,  passing  right  through  the 
cart  body  ;  the  tread  of  the  tyre  is  flat,  and 
it  is  essential  to  have  a  good  runway.  The 
handle  is  reversible,  and  the  whole  cart  is 
designed  to  discharge  its  contents  in  a 
second  or  so.  For  elevator  work,  in  con- 
junction with  a  special  form  of  tower 
described  in  the  next  paragraph,  the  cart 
can  be  fitted  with  legs. 

Hoisting  appliances  are  frequently  neces- 
sary, and  their  nature  will  depend  some- 
what upon  the  design  of  the  building.  Some- 
times it  is  convenient  for  the  concrete  to 
pass  straight  from  the  machine  mixer  into 
cylindrical  steel  buckets,  which  are  then 
hoisted  by  a  steam  crab  or  other  suitable 
means  as  required.  Another  arrangement 
is  to  instal  a  bucket  elevator  of  the  endless- 
chain  type,  there  being  sprockets  at  the 
lowest  and  highest  points  which  drive  the 
chains  to  which  the  buckets  are  attached  ; 
these  buckets  need  to  be  very  strongly  made 
and  have  strengthened  rims.  Other  hoist- 
ing arrangements  as  used  in  general  building 
work  will  frequently  be  found  useful,  and 
these  do  not  need  description  here.  Space, 
however,  must  be  found  for  mention  of  the 
Ransome  tower,  which  has  been  specially 
designed  for  concrete  work.  The  bucket 
used  is  a  modification  of  the  one  previously 
illustrated,  and  is  shown  by  Fig.  48.  It 
has  trunnions  which  are  carried  in  journals 
at  the  bottom  of  a  steel  frame,  which  slides 
up  and  down  between  wooden  guides  in  a 
well-braced  tower  (see  Fig.  49).  The 
bucket  is  carefully  adjusted  for  balance  by 
means  of  stops  until  it  has  a  slight  tendency 
to  tilt  forward.  When  loaded,  it  is  pulled 
up  inside  the  tower  with  its  nosepiece  press- 
ing against  a  front  guide,  and  when  the 
bucket  arrives  at  a  point  in  the  tower  where 
the  front  guide  has  been  cut  away  to  leave 
a  space,  as  shown  at  B,  Fig.  49,  it  auto- 
matically tilts  forward  and  discharges  its 
contents  into  a  bin  from  which  the  concrete 


is  drawn  off  by  means  of  gates  into  barrows 
or  handcarts,  or  in  some  other  suitable  way 
is  carried  to  the  work.  The  hoist  tower  is 
constructed  of  light  timber,  and,  built  accord- 
ing to  Fig.  49,  it  will  accommodate  the 
smallest  size  of  Ransome  bucket,  the 
dimension  A  being  30£  in.,  whereas  in 
the  three  larger  sizes  this  dimension  is 
respectively  37  in.,  44  in.,  and  50|  in.,  the 
other  dimensions  of  the  tower  varying 
accordingly.  It  will  be  realised  that  the 
arrangement  is  one  of  great  ingenuity, 
and,  at  the  same  time,  simplicity.  The 
mixer  discharges  a  batch  into  a  bucket  at 
the  lowest  point  of  the  system.  The  friction 
crab  hoist  shown  to  the  left  of  the  mixer 
operates  a  rope  or  chain,  by  means  of  which 
the  bucket  is  raised  until  it  automatically 
tilts  forward  and  discharges.  The  bucket, 
on  descending  automatically,  rights  itself 
and  comes  to  rest  on  a  rubber  cushion  in 
readiness  for  the  next  batch.  The  bucket 
may  take  other  forms,  and  may  embrace  an 
arrangement  for  dumping  through  the 
bottom  when  an  attendant  raises  a  handle 
or  lever. 

Gravity  System. — The  "  gravity  sys- 
tem "  of  placing  concrete  has  lately  come  to 
the  fore.  Briefly,  it  consists  in  hoisting  the 
concrete  by  means  of  a  skip  and  pouring  it 
into  a  hopper  supported  at  the  top  of  a 
skeleton  tower.  From  this  hopper  the  con- 
crete passes  as  required  through  pipes  which 
discharge  the  concrete  directly  into  the 
forms.  This  system  has  been  used  success- 
fully for  all  classes  of  work,  including  an 
eight-story  office  building  and  long  bridges, 
in  one  case  the  concrete  having  been  con- 
veyed by  gravity  a  distance  of  502  ft. 

LAYING  AND  POURING  CONCRETE 

It  is  desirable  to  deposit  concrete  in  the 
forms  or  moulds  as  gently  as  possible,  and 
in  such  a  way  as  will  allow  of  thorough 
compacting,  a  result  which  will  be  facilitated 
by  working  with  a  spade  or  punner,  until 
the  surplus  water  appears  on  the  surface. 
Gentle  punning  consolidates  the  concrete 
and  is  of  the  utmost  value  in  producing 
good  work.  Of  course,  punning  must  not 
continue  after  the  cement  has  begun  to  set. 

The  depth  to  which  the  concrete  is 
deposited  obviously  depends  entirely  upon 
the  nature  of  the  job.  To  say  that  common 
sense  should  be  employed,  and  the  concrete 
never  deposited  so  thickly  that  there  is 
doubt  as  to  whether  the  reinforcement  is  in 


CONCRETE:     MATERIALS    AND    MIXING 


49 


contact  with  the  concrete  at  every  part,  is 
better  than  to  specify  layers  of  any  par- 
ticular thickness.  Much  has  been  said  as 
to  whether  concrete  may  properly  be  tipped 
from  a  height  into  its  final  resting-place, 
such  a  method  having  been  thought  at  one 
time  to  be  necessary  in  order  to  consolidate 
the  material.  The  method  is  not  harmful 
where  the  vertical  distance  or  "  head "  is 
only  a  few  feet,  but  it  is  not  to  be  recom- 
mended when  the  distance  is  any  more  than 
that,  as  there  is  a  tendency  for  the  heavier 
ingredients  to  separate  from  the  bulk  of  the 
concrete. 

The  very  greatest  care  should  naturally 
be  exercised  to  prevent  any  interference 
with  the  wet  concrete  after  it  has  been 
deposited  and  rammed.  Any  disturbance 
after  setting  begins  is  detrimental  to  the 
strength. 

It  is  usual  to  cover  up  the  concrete  at 


night,  one  object  of  so  doing  being  to  prevent 
any  violent  rain  washing  some  of  the  cement 
out  of  the  top  surface,  and  another,  in  winter r 
to  prevent  frost  reaching  the  work,  since  it 
appears  that  frost  occurring  before  the 
cement  has  set  exercises  a  disintegrating 
action,  and  influences  the  setting  power  and 
ultimate  strength  unfavourably. 

When  depositing  concrete  in  layers,  it  is 
important  to  secure  a  good  bond  between 
the  old  and  the  new  material.  When  there 
has  been  any  length  of  time  between  the  two 
layers,  it  is  necessary  to  wet  the  old  work, 
hack  it  over,  and  sweep  clean.  Many 
authorities  take  the  precaution  of  treating 
the  first  layer,  after  hacking  and  sweeping, 
with  cement  grout,  so  as  to  ensure  a  strong 
bond.  When  one  layer  follows  the  preceding 
one  at  a  close  interval,  all  that  is  necessary 
is  to  see  that  the  surface  of  the  concrete 
already  deposited  is  wet. 


Steel 


Composition  of  Steel. — Steel  consists  of 
iron  containing  carbon  in  the  form  of  iron 
carbide,  the  proportion  of  which,  and  the 
manner  in  which  it  is  held,  largely  deter- 
mining the  character  of  the  steel.  Other 
elements  are  also  present.  Steel  always 
contains  manganese,  and  also  small  amounts 
of  silicon,  sulphur  and  phosphorus.  The 
proportions  of  the  two  latter  are  important, 
and  only  traces  not  exceeding  0-04  per  cent, 
are  permitted  in  structural  steel.  The  table 
given  below  indicates  the  relative  composi- 
tion of  the  various  forms  of  iron  in  use. 


to  the  method  by  which  they  have  been 
produced.  They  are  cast,  not  forged,  into 
shape,  and  the  metal  is  not  malleable  in 
the  above  sense.  The  bending  and  twisting 
of  wire  used  in  reinforced  concrete  work 
for  tying  the  bars  or  making  meshed 
wire  supports  requires  that  the  metal  to  be 
used  shall  possess  the  property  of  malle- 
ability. 

Tenacity. — The  resistance  to  rupture  by 
a  stretching  force  is  known  as  tenacity. 
It  is  determined  by  subjecting  a  piece  of 
the  metal  of  known  sectional  area  to  a 


Silicon 

Carbon 

Sulphur 

Phosphorus 

Graphite 

Combined 

Cast  iron  (castings)* 
Wrought  iron 
Mild  steel 

1—3-5 

tr.—  0-1 
tr.—  0-3 

1—3 

0-15—0-9         0—2 
tr.—  0-25     tr.—  0-1 
0-1—0-5      0-3—1-0 

0—0-2 
tr.—  0-1 
tr.—  0-04+ 

0—1-5 
tr.—  0-2 
tr.  —  0-04| 

Harder  steel 

tr.—  0-3 

— 

0-6—0-8    1      ditto 

ditto 

ditto 

Cutlery  steel 

— 

— 

0-5—1-5       tr.—  0-3 

trace 

trace 

Not  pig  iron  or  malleable  cast  iron, 
f  For  rails  this  may  be  0-06-0-07. 


Alloy  steels  contain,  in  addition  to  carbon, 
other  metals  which  give  their  name  to  the 
steel — nickel,  chromium,  tungsten,  molyb- 
denum, vanadium,  and  manganese — and 
each  of  these  alloy  steels  has  some  special 
character. 

It  will  be  seen  that  the  tough,  tenacious, 
and  malleable  forms  of  iron  are,  with  the 
exception  of  alloy  steels,  those  containing 
only  small  amounts  of  foreign  bodies  other 
than  carbon,  and  that  as  that  element 
increases,  the  hardness  and  elasticity  increase. 
Cutlery  and  tool  steels  contain  the  larger 
amounts  of  carbon. 

Malleability. — No  confusion  should  exist 
as  to  the  term  "  malleable  "  applied  to  iron 
and  steel.  Metal  that  can  be  forged,  rolled, 
bent  and  worked  at  a  red  heat,  and  to  a 
less  extent  in  the  cold,  is  malleable.  The 
term  is  often  applied  without  qualification 
to  certain  kinds  of  castings  which  have  had 
their  brittleness  removed  by  treatment. 
The  composition  of  these  varies  according 


gradually  increasing  stress  in  a  testing 
machine  till  rupture  takes  place.  The  force 
applied  is  indicated  by  the  machine.  In 
Great  Britain,  tenacity  is  usually  stated  in 
pounds  or  tons  per  square  inch  of  sectional 
area.  If  a  test-piece  of  0-7979  in.  diameter 
broke  when  subjected  to  a  load  of  30,000 
lb.,  the  tensile  strength  of  the  material 
would  be  60,000  lb.  or  26-78  tons,  since  the 
bar  is  half  a  square  inch  in  sectional  area. 
The  tensile  strength  varies  with  the 
composition,  and  the  treatment  of  the 
metal.  In  carbon  steels  that  have  been 
heated  above  900°  C.  and  allowed  to  cool 
in  the  air — normalised — the  tenacity  in- 
creases as  the  percentage  of  carbon  rises,  till 
it  attains  a  maximum  of  61  tons  at  about 
1-2  per  cent,  carbon.  Steel  that  has  been 
annealed  by  heating  the  metal  to  redness  and 
allowing  to  cool  very  slowly  has  a  lower 
tenacity.  The  maximum  of  about  36  tons 
is  reached  with  0-9  per  cent,  carbon,  above 
which  it  falls  away  again.  The  graph 


STEEL 


presented  by  Fig.  50,  and  the  figures  on  the 
preceding  page  are  given  by  Prof.  Arnold. 

Increasing  the  rapidity  of  cooling — by 
quenching  from  redness — may  raise  the 
tensile  strength,  but  affects  also  other 
properties.  The  smaller  the  amount  of 
carbon  present,  the  less  is  the  effect  produced 
by  varying  the  rate  of  cooling.  Very  soft 
steels,  containing  little  carbon,  are  not 
greatly  affected,  as  will  be  observed  from 
the  diagram  just  referred  to. 

The  divergence  in  the  indicated  strengths 
between  the  normalised  and  annealed  speci- 
mens becomes  more  marked  as  the  MAX. 
percentage  of  carbon  increases.  |NTToSNSs 
The  quenching  effect  also  follows  the  PER  so  IN. 
same  course,  and  is  much  greater  60 

with  the  higher  carbon  steels.  They  at 
the  same  time  become  much  harder  and 
more  brittle.  The  hardening  effect  50 
becomes  sensible  above  0-35  per  cent., 
and  with  045  per  cent,  is  quite  distinct. 
With  1  per  cent,  the  metal  becomes 
brittle  and  very  hard. 

Under  ordinary  treatment  a  percen- 
tage   of    carbon    below  0-8   per  cent. 

'^n 

raises   the   tensile   strength,    but    does 
not    make    the    metal    brittle.      Steel 
containing  from  0-7  to  0-85  of  carbon, 
although  its  ductility  has  been  greatly     20 
reduced,  is  quite  suitable  for  reinforced 
concrete  construction,  as  it  is  still  suf- 
ficiently ductile  to  be  free  from  liability      10 
to  breaking  under  ordinary  conditions  ; 
it  is,  however,  much  stiffer  and   more 
rigid  and     elastic   than    metal    having       o 
a  lower  carbon  content.     Do  steels  with 
higher  carbon  contents,  used  in  plain 
bars,  and  not  ridged,  notched  or  other- 
wise "  deformed,"   secure  a  more  per- 
manent grip  on  the  concrete  than  do 
steels  with  lower  carbon  contents  ?     It  is  a 
moot  point. 

Ductility. — The  power  of  being  drawn 
out  or  extended  is  known  as  "  ductility," 
and  it  is  indicated  by  the  extent  by 
which  the  test  piece  in  a  tensile  test 
elongates  before  fracture.  This  elongation 
is  expressed  as  a  percentage  of  the  original 
length  of  the  piece.  Before  testing,  two 
centre- punch  or  other  marks  are  made  on 
the  piece,  at  a  carefully  measured  distance 
apart.  After  fracture  the  two  portions 
are  laid  together  and  the  distance  again 
measured.  The  increase  is  calculated  in 
percentage  terms  of  the  original  length. 
In  stating  elongation,  the  length  of  the  test- 


piece  must  be  given,  since,  owing  to  the 
extensive  thinning  of  the  bar  at  the  point 
of  fracture,  much  of  the  elongation  is  con- 
fined to  that  portion  in  which  the  fracture 
occurs.  This  is  the  case  with  both  short 
and  long  test-pieces,  but  on  the  short  pieces 
the  local  elongation  represents  a  higher  per- 
centage of  the  original  length. 

Ductility  decreases  as  the  carbon  increases, 
as  the  graph  (see  Fig.  51  on  next  page) 
by  Prof.  Arnold  shows.  Here,  again,  the 
effect  of  rapid  cooling  is  plainly  indicated. 
Carefully  annealed  material  shows  a  mini- 


>A 

/ 

/ 

^ 

/ 

7^ 

/ 

/ 
£L 

^*^ 

^ 

«/ 

<y  ^ 

^ 

/ 

\ 

A 

*T 

A 

•  25%          '5%          -75%           \'Q°l        \'25%       \'5% 

CARBON 

Fig.  50. — Graph  Showing  Influence  of  Carbon  on 
Tenacity  of  Steel  :  N,  normalised  specimens, 
and  A,  annealed  specimens 

mum  ductility  at  about  0-89  per  cent,  carbon, 
after  which  the  ductility  rises.  With  normal- 
ised metal  the  decrease  continues  to  little  more 
than  2  per  cent,  at  147  per  cent,  carbon. 

The  difference  in  the  behaviour  of  the 
metal  in  the  two  states  results  from  the 
difference  in  the  manner  in  which  the  car- 
bide is  held.  The  carbide  dissolves  and 
diffuses  through  the  metal  when  heated. 
If  slowly  cooled  it  separates  completely 
from  the  iron  during  the  cooling.  More 
rapid  cooling  prevents  the  separation  from 
being  completed,  while  quenching  may 
prevent  the  separation  altogether.  Thus  the 
properties  of  the  metal  vary  with  the  rate 
of  cooling  and  the  amount  of  carbide. 


REINFORCED    CONCRETE 


Prom  the  above  it  will  be  seen  that  for 
material  that  must  be  subjected  to  severe 
treatment  in  forging  and  use,  such  as  boiler 
plates,  rivets,  etc.,  only  soft,  low-carbon 
steels  can  be  applied.  Steel  containing 
more  carbon  would  be  too  hard  and  not 
sufficiently  malleable  and  ductile,  so  that 
cracking  or  fracture  might  occur.  With 
materials  which  are  not  subjected  to  severe 
treatment  and  where  high  tensile  strength, 
elasticity,  and  rigidity  are  more  important, 
higher  proportions  of  carbon  are  allowable. 
Thus,  with  0-9  per  cent,  carbon,  a  tensile 
strength  of  51  tons  per  square  inch  and 


ELONGATION 
PER  CENT 

ON    2" 


50 


40 


30 


20 


10 


•25% 


•5% 


•75% 


'25% 


Fig.   51. — Graph  Showing  Influence  of  Carbon  on 
Ductility  of  Steel 


an  elongation  of  14  per  cent,  are  obtained, 
the  latter  being  somewhat  low,  but  higher 
than  in  the  case  of  much  wrought  iron,  the 
tensile  strength  of  which  is  less  than  that 
of  the  steel. 

Elasticity. — This  is  the  power  of  the 
steel  to  recover  its  original  shape,  and  it 
is  influenced  by  the  manner  in  which  the 
carbon  is  held  in  the  metal. 

Limit  of  Elasticity. — This  term  is 
applied  to  the  force  which  will  produce  a 
permanent  set  or  extension  of  the  metal 
under  tensile  test ;  that  is,  the  point  at 
which  the  metal  ceases  to  be  perfectly 
elastic.  It  is  usually  about  half  the  total 
strength.  The  metal  will  continue  to 
stretch  on  the  further  application  of  force, 
but  the  extension  is  not  then  proportional 


to  the  force  applied,  and  the  metal  does 
not  recover  its  original  dimensions  on  the 
removal  of  the  stress,  but  is  permanently 
strained. 

Testing  Steel. — Fig.  52  shows  test- 
pieces.  They  are  carefully  machined  to 
shape,  marked  by  means  of  a  centre -punch 
for  determining  elongation,  and  the  ends, 
firmly  gripped  by  the  jaws  of  the  machine. 
Force  is  gradually  applied  to  pull  the  ends 
apart  and  the  piece  carefully  watched.  At 
a  certain  point,  a  change  in  the  outer 
skin  may  be  observed.  With  a  lever 
machine  the  sudden  elongation  may  cause 
the  lever  to  drop.  This  point  is 
the  commercial  elastic  limit.  Fur- 
ther application  of  force  produces 
further  extension,  and  ultimately 
a  point  is  reached  when  the  in- 
creasing load  causes  the  piece, 
which  up  to  this  point  has  been  ex- 
tending uniformly  over  its  whole 
length,  to  begin  to  contract  locally, 
and  rapidly  to  diminish  in  sectional 
area  ;  in  this  region  the  piece  ulti- 
mately breaks.  The  contraction  in 
area  is  generally  stated  as  a  percent- 
age of  the  original  area.  The  frac- 
ture on  examination  should  be 
uniform,  free  from  flaw  or  defect, 
and  silky  or  finely  granular.  Kound 
pieces  should  show  cup  and  cone 
formation  if  soft.  Fracture  should 
occur  near  the  middle,  and  with 
uniform  material  there  should  be 
'*5*  only  one  region  of  contraction.  Tests 
in  which  this  does  not  occur  or 
which  break  very  near  one  end  are 
not  satisfactory.  The  effective 
length — that  is,  the  distance  between  the 
marks — varies  from  2  in.  to  10  in.  ;  but 
usually  2-in.,  6-in.,  8-in.,  or  10-in.  test 
pieces  are  employed. 

SPECIFICATION  FOR  STEEL  REINFORCE- 
MENT 

The  following  is  a  suitable  specification 
for  steel  reinforcement : 

The  steel  used  shall  be  manufactured 
by  the  Open  Hearth  process,  and  shall 
be  of  the  very  best  description,  free  from 
blisters,  scales,  laminations,  and  shall  be 
of  the  size  and  weight  specified  (a  vari- 
ation of  2£  per  cent,  either  way  is  usually 
allowed). 

The  chemical  analysis  shall  show  —  per 
cent,  carbon,  and  not  more  than  the  fol- 


STEEL 


53 


lowing    percentages 
sulphur  : 

Phosphorus 
Sulphur 


of     phosphorus     and 

.  0-06  (maximum) 
.  0-04  (maximum) 


On  this  assumption  a  force  of  1  Ib.  would 
extend  or  reduce  the  length  of  a  test-piece 

of  the  oriinal 


No  allowance  above  these  figures  must  be 

made.      The  steel  shall  have   an   ultimate 

tensile  strength  of  not 

less     than    28    to    32 

tons    per   square   inch, 

and   an    elongation   of 

at  least    15   per    cent. 

on  an  8-in.  length.    No 

welds  shall  be  made  in 

the     steel    under    any 

•circumstances.      When 

forging     is      necessary 

special   attention   shall 

be  paid  to  the  smith's 

work.     No  overheated  or  burnt  steel  to  be 

used.       All    bending    (where    possible)    of 


CERTIFICATE  OF  TESTS 

The  following  is  a  sample  certificate   of 


t  ~ 


Fig.  52. — Test  Piece  Before  and  After  Stretching 


tests  for  steel  used  in  reinforced  concrete 
construction  : — 


CERTIFICATE    OF   TEST 
Messrs.  X. 

Tests  of  Mild  Steel  to  your  order. 
For  delivery  to  A.  B. 


No.  of  Sample 

Original  size 

Contraction  of  Area 

Elongation 
on  8  inches 

Ultimate 
stress 

Elastic  limit 

^    £ 

1 

l-i 

|i 

•3  4 
5  8 

£  S  1 

£8* 

1*1 

j 

S3 

Hj 

S't 

fi 

§'". 

1J  Rounds     . 
1  X  i  Flats  . 
Ditto    . 
1  inch  square 

1-12 
1-01  x  0-495 
1-00  x  0-505 
1-OOx  0-99 

0-985 
0-5 
0-505 
0-99 

0-76  Diam. 
0-79  x  0-295 
0-75  X  29 
0-7    X  0-69 

0-454 
0-232 
0-217 
0-482 

54 
53-5 
57-0 
51-3 

10-22 
10-24 
10-16 
10-25 

27-7 
28-0 
27-0 
28-2 

28-6 
14-6 
14-4 

29-8 

29 
29-2 
28-5 
29-9 

18-64 
9-75 
10-10 

20-8 

18-9 
19-5 
19-9 
21 

1  X  J  Bend  tests  satisfactory. 

Cold  Bend  tests  satisfactory. 

1  X  J  Flats  cold  bend  tests  satisfactory. 


small  metal  to  be  done  cold.  Larger 
metal  to  be  heated  to  a  dull  cherry 
red.  The  bending  force  must  be  applied 
gradually  and  regularly.  Before  use,  all 
steel  must  be  clean  and  free  from  scale  and 
rust.  No  oil  or  paint  must  be  used  on  the 
steel. 

Modulus  of  Elasticity.  —  This  is  the 
force  in  pounds  that  would  be  required  to 
extend  a  bar  to  double  or  compress  it  to 
half  its  original  length  on  the  assumption 
that  it  remained  perfectly  elastic  throughout. 
It  is  frequently  called  the  coefficient  of 
elasticity  ;  thus  for  steel  the  modulus  is 
about  30,000,000  Ib.,  and  the  coefficient 

*s  ^a^v  constant  for  open- 


JO  000  000' 
liearth  steel  with  varying  carbon  contents. 


Fatigue. — When  subject  to  varying  and 
fluctuating  stresses,  metals  are  liable  to 
alteration.  The  tensile  strength,  ductility, 
and  toughness  are  reduced.  In  some  cases 
this  is  very  pronounced,  and  the  strength 
may  fall  below  the  ordinary  working  load 
and  fracture  take  place.  The  alteration 
under  ordinary  conditions  takes  place  slowly 
in  steels  that  are  free  from  injurious  con- 
stituents and  of  uniform  texture.  The 
elasticity  generally  depends  on  the  latter 
condition,  and  metals  of  high  elasticity  are 
less  liable  to  fatigue.  The  elastic  limit  in 
the  case  of  steel  is  higher  than  for  wrought 
iron,  and  its  resistance  to  internal  change  is 
greater. 

Toughness. — This  may  be  defined  as 
the  resistance  to  fracture  by  bending  beyond 


54 


REINFORCED    CONCRETE 


the  elastic  limifci  It  is  especially  necessary 
in  steel  that  must  be  bent  or  twisted.  Soft 
malleable  steel,  of  uniform  texture  and 
character,  is  toughest ;  but  very  much 
depends  on  the  heat  treatment  the  steel 
has  received. 

Hardness. — This  is  difficult  to  determine 
with  accuracy,  and  for  reinforced  concrete 
work  is  only  of  secondary  importance.  It 
is  dependent  on  the  heat  treatment  and  rate 
of  cooling  of  the  metal,  and  on  the  percentage 
of  carbon  present  in  the  steel.  Reference 
to  the  following  table,  and  a  consideration 
of  the  uses  of  steels  of  varying  composition, 
will  illustrate  the  latter  point,  and  the  study 
of  the  effects  of  carbon  to  be  dealt  with 
later  will  explain  the  cause  of  the  increased 
hardness  produced  by  quenching  from  a 
red  heat. 


Steel 

Carbon  contents 

Soft    malleable    metal 
plates  and  rivets   . 


for 


For  ordinary  wire  for  ropes 

Hard  wire  for  guide  ropes  . 

Rails        .... 

Rolled  sections 

Hard  open-hearth 

Tool  steel  for  dies  and  steel- 
ing purposes  (axes  and 
plane  irons,  die  temper)  . 

Setts,  minting  dies,  smith's 
tools  (sett  temper) 

Cold  chisels,  miners'  drills, 
large  punches  (chisel 
temper) 

Circular  cutters,  taps, 
rimers,  large  turning 
tools  and  drills,  screwing 
dies  (punch  temper) 

Turning  tools    . 

For  small  tools,  sawfiles,  etc. 

For  razors  and  special 
purposes 


01  —0-3 
0-2  —04  Ma. 
0-15—0-3 
045—0-75 
0-3  —0-6 
0-22—0-25 
0-6  —0-8 


0-75 

0-825 

1-00 


1-125 

1-25 

1-375 

1-5 


matter  included  in  the  steel.      Overworking 
the  steel  may  produce  lamination. 

Usually  with  increased  diameter  a  smaller 
demand  is  made  for  elongation.  For  an 
increase  of  £  in.  above  f  in.  a  deduction 
of  1  per  cent,  is  made,  for  decreases  of  j\  in. 
below  yV  a  deduction  of  2|  per  cent,  from 
the  specification. 

Bending  Tests. — These  consist  of  bend- 
ing the  metal  as  per  specification  upon  itself. 
Rivet  steel  is  tested  at  full  size  as  rolled. 
It  must  bend  flat  on  itself — through  180° 
— without  fracture  on  the  outside  of  the 
bend.  All  material  less  than  f  in.  is  tested 
at  full  thickness,  but  above  that  thickness  the 
specimen  used  is  £  in.  thick,  and  if  possible 
1£  in.  wide.  Soft  steel  must  bend  over  on 
itself — through  180° — without  fracture  at 
any  part  of  the  bend.  Medium  steel  must 
bend  through  180°,  round  a  diameter  equal 
to  the  thickness  of  the  metal,  so  that  the 
two  limbs  are  separated  by  a  distance  equal 
to  the  thickness  of  the  test,  without  showing 
sign  of  fracture  on  either  side.  For  works 
tests,  the  pieces  are  often  doubled  closely 
together.  Tests  after  quenching  from  a 
red  heat  are  somewhat  less  severe,  the 
distance  between  the  limbs  being  equal  to 
1|  times  the  thickness  of  the  metal. 

The  importance  of  a  high  elastic  limit 
will  be  appreciated  when  the  effect  of 
subjecting  to  tensile  strain  a  smooth  round 
bar,  one  end  of  which  is  embedded  in  con- 
crete, is  considered.  In  such  circumstances 
the  load  may  be  increased  till  the  elastic 
limit  is  reached  without  in  any  way  per- 
manently lessening  the  grip  of  the  concrete 
on  the  metal.  When  the  elastic  limit  is 

TENSILE  STRENGTH  AND  ELASTIC  LIMIT 
OF  STEEL  IN  TENSION  AND  COMPRESSION 
IN  POUNDS  PER  SQUARE  INCH.* 


High-speed  steels  containing  tungsten, 
chromium,  and  other  metals  to  render  the 
metal  self-hardening  contain  only  0-5 — 0-7 
per  cent,  carbon. 

Referring  to  the  specification  already 
given,  freedom  from  blister  may  be  secured 
by  using  ingots  free  from  blow  holes.  Scales 
and  laminations  result  from  faulty  treatment 
in  the  forging  or  rolling,  or  to  some  foreign 


Percentage 

Ultimate 

Elastic 

Elastic 

of                 tensile 

limit  in 

limit  in 

Carbon 

strength 

tension 

compression 

0-14 

62,944 

41,955 

39,536 

0-19 

68,096 

47,084 

43,008 

046 

75,712 

49,056 

48,944 

0-55 

80,416 

46,995 

49,772 

0-66 

89,600 

53,244 

53,648 

0-78 

92,064 

53,312 

53,648 

0-82 

102,816 

57,003 

63,168 

0-87 

104,608 

61,017 

56,000 

0-96 

118,048 

69,216 

71,120 

*  Quoted  from  Twelvetrees'  "  Concrete-Steel." 


STEEL 


55 


reached,  the  permanent  extension  with  the 
accompanying  thinning  of  the  bar  would 
release  it  as  far  as  contraction  extends,  and 
this  would  probably  progress  along  the  bar 
by  the  repetition  of  the  stress. 

Shearing  Strength. — The  value  of  this 
factor  is  very  largely  influenced  by  the 
structure  of  the  steel  developed  in  rolling 
and  the  heat  treatment.  Usually  it  amounts 
to  70  to  75  per  cent,  of  the  ultimate  tensile 
strength. 

MODULI   OF    ELASTICITY    OF    STEEL   WITH 

DIFFERENT    PERCENTAGES      OF      CARBON 
IN   1,000,000-LB.    UNITS. 


Carbon 

Tension 

Compression 

Bending 

Mean 

0-19 

30-8 

37-0 

29-1 

32-9 

0-46 

32-0 

32-7 

29-3 

31-8 

0-54 

30-6 

36-1 

28-8 

32-4 

0-66 

32-4 

35-7 

32-1 

33-6 

0-78 

33-5 

32-4 

30-1 

32-4 

0-87 

31-0 

31-5 

30-4 

31-1 

0-96 

30-9 

32-7 

29-2 

31-2 

Resistance  to  Alternating  and  Re 
peated  Stresses. — For  all  structural  pur- 
poses the  permanence  of  the  steel  is  of  the 
highest  importance.  Repeated  stresses  in- 
duce a  brittle  condition  which  is  not  in- 
dicated by  the  testing  machine.  The  piece 
breaks  as  though  it  were  absolutely  brittle. 
Tests  of  brittleness  are  made  by  bending 
small  prepared  test-pieces  in  opposite  direc- 
tions through  a  small  angle  by  means  of  a 
suitable  machine.  The  number  of  bendings 
before  fracture  occurs  varies  with  the  com- 
position and  the  treatment  the  steel  has 
received.  Mr.  Stead  gives  the  following 
figures  for  steel  containing  044  per  cent, 
carbon : 


Normal 
Overheated 
Reheated  . 
Annealed  . 
Sorbitic 


1,432,500 
844,950 
2,080,440 
1,971,000 
3,517,200 


The  effect  of  subjecting  bars  to  mechanical 
treatment,  and  the  consequent  variation  in 
the  results  obtained,  is  evidenced  in  the 
table  given  below.  The  bars  tested  had 
been  twisted  at  one  end  while  cold. 


The  rate  of  alternation  has  a  very  con- 
siderable effect.  Prof.  Arnold  has  shown 
that  the  resistance  to  rupture  is  inversely 
proportional  to  the  rate  of  alternation. 
Steel  that  has  become  brittle  under  such 
conditions  cannot  be  restored  by  annealing, 
neither  can  it,  indeed,  by  any  other  heat 
treatment. 


Description 
of  Bar 

Original 
size  in 
inches 

Orioinal 
area  in  sq. 
inches 

Elastic  stress 
•per  sy.  inch 

Ultimate 
strength  -per 
sq.  inch 

Ratio  of 
Elastic  to 
Ultimate 

Contraction 
in  area  at 
fracture 

Extension 
in  8  inches 

Extension 
in  10  inches 

5-88  twists  per 
foot 

Plain  end 

|  square 

•377 

X 

•382 

0-144 

43,000  Ib. 
=  19-2  tons 

58,847  Ib. 
=26-3  tons 

73-1 

71-5 

27-5 

26-2 

Silky 

Twisted  end 

do. 

do. 

59,700  Ib. 
=26-7  tons 

86,250  Ib. 
=38-5  tons 

69-2 

59-7 

6-7 

6-0 

do. 

4  '21  twists  per 
foot 

Plain  end 
£  square 

0-5 

X 

0-5 

0-25 

44,000  Ib. 
=  19-6  tons 

62,400  Ib. 
=27-9  tons 

70-5 

67-6 

28-7 

27-6 

do. 

Twisted  end 

do. 

do. 

78,400  Ib. 
=35-0  tons 

88,480  Ib. 
=39-5  tons 

88-6 

56-4 

7-2 

6-3 

do. 

3-  15  twists  per 

foot 

Plain  end 
f  bar   . 

0-613 

X 

0-615 

0-377 

31,500  Ib. 
=  14-1  tons 

53,475  Ib. 
=23-9  tons 

58-9 

69-8 

35-5 

33-9 

do. 

Twisted  end 

do. 

do. 

64,000  Ib. 
=28-6  tons 

73,820  Ib. 
=33-0  tons 

86-7 

63-7 

7-0 

6-0 

do. 

REINFORCED    CONCRETE 


Effects  of  Heat  on  Physical  Pro- 
perties.— Sir  W.  Fairbairn  showed  that 
the  strength  of  rivet  iron  and  boiler  plates 
(best  Yorkshire  iron)  increased  as  the 
temperature  rose  to  435°  F.,  rising  from 
62,720  Ib.  at  60°  F.  to  86,016  Ib.  at  435°. 
At  1290°  F.  the  strength  was  only  35,840  Ib. 
Mild  steel  does  not  so  increase  in  strength. 
Styffe  showed  that  between  212°  F.  and 
392°  F.  the  strength  remained  practically 
unchanged. 

Other  experimenters,  including  Howard, 
Barnaby,  and  Martens,  have  shown  that 
between  60°  F.  and  150°— 250°  there  is  a 
fall  in  tenacity,  followed  by  an  increase  up 
to  450°  or  600°  F.  Beyond  this  point  the 
tenacity  falls,  and  at  1100°  to  1200°  F.  is 
only  about  half  the  original. 

Although  an  actual  increase  in  strength 
may  be  registered,  the  ductility  is  diminished. 
At  about  550°  to  570°  F.  the  metal  shows 
signs  of  brittleness,  and  fracture  occurs 
suddenly  without  elongation.  Eesistance 
to  shock  at  or  above  these  temperatures 
is  therefore  probably  much  diminished. 
This  temperature,  be  it  noted,  is  much  below 
redness. 

The  minimum  length  of  a  bar  to  be  em- 
bedded in  concrete  in  order  to  ensure  its  not 
slipping  under  tension  will  depend  on  the 
adhesion  between  the  two  substances,  the 
elasticity  and  the  area  of  the  metal  surface 
embedded.  Until  the  bar  stretches  the 
adhesion  will  remain  unimpaired.  It  may, 
however,  be  affected  by  unequal  expansion 
of  the  materials,  when  the  composite 
material  is  subjected  to  heat  and  also  to 
internal  stress  of  a  variable  character  such 
as  vibration  and  shock.  Any  advantages 
derived  from  the  shape  of  the  bar  is  due 
either  to  the  increased  area  caused  by  the 
shape  given  or  to  the  irregularity  of  form, 
securing  a  bond  when  the  natural  one  may 
have  been  reduced  or  destroyed. 

The  adhesion  varies  greatly,  and  is  in- 
fluenced by  the  nature  of  the  cement,  the 
amount  of  water  used  and  other  factors. 

Coefficient  of  Expansion. — The  co- 
efficient of  expansion  of  steel  is  0-000066 
for  mild  steel,  and  0-000069  for  hard  steel. 

Other  Elements  Present  in  Steel.— 
Beside  carbon,  steel  also  contains  small 
amounts  of  other  elements. 

Silicon. — Usually  only  traces  of  this  ele- 
ment are  present,  being  the  remains  of  what 
was  present  in  the  pig  iron  used  in  making 
the  steel,  and  which  was  not  eliminated 


during  the  process.  It  is  generally  below 
0-1  per  cent.  In  such  amounts  its  effects  are 
negligible.  Small  amounts  of  silicon  in  steel 
have  the  effect  of  ensuring  sounder  metal 
freer  from  blowholes,  and  it  is  often  added 
for  this  purpose  to  the  metal  before  casting. 
In  larger  amounts,  silicon  has  the  effect  of 
largely  increasing  the  size  of  the  crystal 
grains,  and  with  2-5  per  cent,  the  metal 
presents  on  fracture  an  appearance  more  or 
less  like  spiegel.  Further  increase  produces 
a  fracture  resembling  silicon  iron.  Turner 
found  that  up  to  0-315  silicon  had  no  effect 
on  the  tensile  strength  or  ductility.  With 
0-5  per  cent,  an  increase  of  7  tons  in  the 
tensile  strength  and  a  diminution  of  4  per 
cent,  on  the  elongation  took  place. 

Larger  amounts  of  silicon  have  the 
effect  of  raising  both  the  tensile  strength 
and  the  elastic  limit  of  the  metal. 
With  small  proportions  of  silicon,  no  loss 
of  ductility  occurs,  but  with  amounts  beyond 
2  per  cent,  the  loss  in  ductility  is  very 
marked,  and  with  4  per  cent,  the  ductility  is 
practically  nil.  The  metal  remains  malleable 
if  the  carbon  content  is  low,  until  more  than 
6  per  cent,  of  the  element  is  present.  Beyond 
this  it  is  brittle.  With  increased  carbon  less 
silicon  will  produce  brittleness.  Silicon  up 
to  2  per  cent,  has  no  effect  on  the  hardening 
of  steel  by  quenching  in  water  even  from 
a  welding  heat.  More  than  this  renders  the 
steel  somewhat  stiffer,  but  not  harder. 
Silicon  steels  are  being  used  for  tram  rails 
and  in  work  where  high  elasticity  is  required. 
It  has  been  applied  to  the  making  of  springs 
for  motor-cars  and  other  vehicles  For  rails 
0-3  per  cent,  with  0-5 — 0-6  per  cent,  carbon 
and  1  per  cent,  manganese,  and  for  the  latter 
0-8 — 0-9  per  cent,  silicon  and  0-7  per  cent, 
carbon  and  04  per  cent,  manganese.  With 
the  latter  alloy,  very  careful  treatment  is 
necessary  to  develop  the  mechanical  pro- 
perties. Carefully  annealed,  the  tensile 
strength  is  about  50  tons,  with  an  elastic 
limit  of  30  tons,  and  14  to  18  per  cent, 
elongation.  After  quenching  at  about  900° 
C.  and  reheating  to  500°  C.  the  tensile 
strength  is  85 — 95  tons,  elastic  limit  63 — 67 
tons,  with  an  elongation  of  5  to  12  per  cent. 
The  table  on  p.  57  is  quoted  from  Greenwood 
and  Sexton's  "  Steel,"  and  shows  the  chief 
mechanical  properties  of  the  metal. 

Manganese  is  a  constant  constituent  of 
mild  steel.  Iron  free  from  or  containing 
little  carbon  cannot  be  melted  and  cast 
without  becoming  burnt.  Manganese  is 


STEEL 


57 


Analysis 

Unannealed 

Annealed 

Per  cent. 

Tons  per  sq.  in. 

Per  cent. 

Tons  per  sq.  in. 

Per  cent. 

C. 

Si.          T.S. 

E.L. 

E. 

C. 

T.S. 

E.L. 

E. 

C. 

0-14 

0-24 

33 

22 

30 

54 

25 

15 

37 

61 

0-18 

0-73 

34 

25 

29-5 

54-5 

29-5 

19 

34 

52-5 

0-19 

1-6 

37-5 

28 

31 

50 

33 

25 

35 

54-5 

0-20 

2-18 

39-5 

31 

18-5 

28 

34 

25-5 

36-5 

60 

0-20 

2-7 

42-5 

32 

17-5 

24 

32 

24 

6 

6-5 

0-21 

3-4 

47-5 

35 

11 

14 

39 

30 

9 

9 

0-25 

4-2 

49 

45 

0-004 

0-2 

38 

none 

0-64 

0-98 

026 

5-5 

48 

none 

0-3 

0-7 

25 

25 

0-37 

0-5 

added  in  the  form  of  spiegeleisen  or  ferro- 
manganese  to  deoxidise  it  and  thus  restore 
malleability.  Its  tendency  is  to  make  the 
metal  harder  and  to  raise  the  tensile  strength 
slightly,  but  to  reduce  the  ductility.  It 
may  perhaps  correct  to  some  extent  the 
effect  of  sulphur  in  producing  red  shortness. 
For  structural  steel  the  amounts  present 
vary  from  0-2  to  1  per  cent.  The  latter  is 
allowed  in  rails  and  is  occasionally  exceeded. 
Generally  the  amount  is  below  0-6  per  cent. 

Phosphorus. — This  element  is  present  in 
steel  as  the  phosphide,  containing  15-57 
per  cent,  of  phosphorus  ;  that  is,  phosphorus 
produces  nearly  6J  times  its  weight  of  the 
compound,  and  the  effects  must  be  judged 
in  this  light.  Its  general  effect  is  to  produce 
cold  shortness,  brittleness,  and  sensitive- 
ness to  shock.  It  produces  a  crystalline 
structure.  The  metal  rolls  well  hot,  and 
its  tensile  strength  and  elasticity  are  raised, 
but  the  ductility  is  reduced,  and  may  be 
completely  destroyed.  The  effect  on  the 
ductility  and  toughness  is  intensified  by 
the  presence  of  carbon.  With  amounts 
under  0-1  per  cent,  but  little  change  can 
be  observed  in  the  metal  under  test,  but 
its  sensitiveness  to  shock  may  be  very  greatly 
increased.  Although  its  behaviour  varies, 
its  reliability  in  this  respect  is  destroyed. 
It  is  necessary  therefore  to  specify  a  maxi- 
mum. For  rails  and  similar  purposes  a 
maximum  of  0-08  is  permissible,  and  0-07  is 
usually  specified,  but  0-04  is  often  required 
in  steels  containing  0-4  to  0-6  carbon. 
With  higher-carbon  steels  less  phosphorus 
is  allowable,  and  the  lower  figure  should  be 
adhered  to.  For  tool  steels  the  phosphorus 
should  not  exceed  0-02  per  cent. 

Sulphur. — This  element  also  exercises, 
even  in  small  quantities,  a  serious  influence 
on  the  properties  of  steel.  It  induces  red 


shortness  when  the  amounts  present  are 
not  sufficient  to  produce  any  effect  on  the 
tenacity  or  ductility  of  the  steel  in  the 
cold.  The  steel  cracks  on  rolling  even  when 
the  sulphur  present  is  less  than  0-1  per  cent., 
and  the  steel  is  therefore  very  unreliable, 
since,  although  externally  satisfactory,  in- 
ternal cracks  and  flaws  may  be  produced 
which,  under  the  conditions  of  application 
— vibration,  variable  temperature,  etc. — 
may  ultimately  weaken  the  steel  below  its 
working  load  and  produce  fracture.  It  is 
most  likely  to  produce  fatigue,  and  its 
presence  destroys  the  welding  power  of  the 
steel.  Not  more  than  0-05  per  cent,  should 
be  allowed  in  any  circumstances. 

Carbon. — As  already  noted  most  of  the 
changes  in  the  mechanical  characters  of 
steel  are  produced  by  the  carbon  present. 
Not  only  does  an  increase  in  the  element 
produce  a  change  in  properties,  but 
different  properties  may  be  presented 
by  steels  containing  the  same  amount,  if 
they  have  received  different  heat  treatment. 
Steel  heated  to  redness  and  quenched 
is  made  harder  to  an  extent  depending 
on  the  amount  of  carbon  present. 
The  same  steel,  if  reheated  and  allowed  to 
cool  slowly,  is  softened.  In  the  softened 
state  it  is  harder  than  iron  free  from  carbon. 
The  tensile  strength  is  raised,  but  the 
ductility  is  lowered.  The  amount  is  not, 
however,  in  proportion  to  the  increase  of 
strength.  This  renders  carbon  the  best 
hardening  agent,  as  it  produces  the  greatest 
increase  in  tenacity  with  the  least  diminution 
of  ductility.  In  hardening,  the  change 
occurs  at  a  definite  temperature  or  at  least 
over  a  very  limited  range  of  temperature. 
If  the  steel  cools  below  this  temperature 
before  quenching,  it  is  not  hardened,  nor  is 
it  softened  unless  this  temperature  is  reached. 


REINFORCED    CONCRETE 


Below  this  temperature  it  is  immaterial 
whether  the  cooling  is  rapid  or  slow.  The 
temperature  is  known  as  the  "  critical 
temperature "  of  hardening,  and  also  as 


Fig.  53. — Photo-micrograph  of  Hard  Steel 
(3  per  cent,  carbon)  Cooled  Slowly 

the  "  recalescence  point."  Steel  of  suitable 
carbon  content  and  volume  actually  becomes 
visibly  hotter  as  it  cools  from  a  red  heat 
when  the  critical  temperature  is  reached. 
Hence  the  term  "  recalescence."  It  marks 
the  temperature  at  which  the  carbon 
contents  of  the  steel  change  their  condition, 
this  coinciding  with  the  hardening  effect. 

Carbon  exists  in  steel  in  chemical  combina- 
tion with  iron.  The  carbide  that  has  been 
separated  has  the  composition  Fe3C,  and  is 
insoluble  in  iron  below  the  recalescence 
point,  but  soluble  above  it.  If  quenched 
from  a  higher  temperature  its  separation  is 
prevented,  and  it  remains  in  solid  solution, 
in  which  condition  it  produces  its  maximum 
effect.  The  condition  it  assumes  after 
or  during  separation  may  seriously  affect 
the  properties.  A  microscopic  examination 
of  polished  and  etched  steel  shows  that 
when  cooled  in  air  the  carbide  has  collected 
into  small  areas,  surrounded  by  uncar- 
burised  iron.  The  size  of  these  areas  in- 
creases with  the  carbon  contents,  and  with 
0-9  per  cent,  carbon  the  whole  surface  is 
uniform.  The  grey  areas  are  seen  under  a 
high  magnification  to  consist  of  alternate 
plates  of  light  and  dark  material.  One  of  the 


components  is  the  carbide — Fe3C  (cementite), 
and  the  other  the  iron  (ferrite)  from  which  it 
has  separated.  If  cooled  very  slowly  the 
structure  is  coarsely  laminated.  Cooled 
more  quickly  it  is  finer,  and  the  laminated 
structure  may  be  very  fine  or  even  granular. 
The  latter  condition,  which  is  stronger  and 
more  elastic,  is  the  sorbitic  structure.  The 
laminated  material  under  inclined  illumina- 
tion often  shows  a  play  of  colour  and  was 
named  by  its  discoverer,  Dr.  Sorby,  pearlite. 
Heated  beyond  the  recalescence  point  the 
cementite  first  dissolves  in  the  iron  (ferrite) 
of  the  pearlite,  and  then  diffuses  through  the 
mass.  On  quenching,  its  condition  remains 
what  it  was  in  the  heated  state,  and  the 
hardening  effect  is  produced. 

If  cooled  slowly,  steels  containing  more 
than  0-89  carbon  first  separate  the  surplus 
cementite,  and  this  continues  till  the  recales- 
cence point  is  reached.  The  structure  is 
shown  at  Fig.  53.  If  less  than  0-89  per 
cent,  carbon  is  present,  the  excess  of  iron 
(ferrite)  first  separates,  and  the  carbide 
concentrates  into  what  ultimately  become 
the  pearlite  areas  (see  Fig.  54),  final  separa- 
tion and  lamination  being  produced  at  the 
recalescence  point.  The  more  complete 
the  separation,  the  softer  the  material  will 
become,  and  the  less  will  be  the  effect  of  the 


Fig.   54. — Photo-micrograph  of  Steel  (Dark 
Areas  show  Pearlite) 

carbon  in  raising  the  tensile  strength  "and 
elasticity.  With  largely  laminated  pearlite 
the  ductility  is  not  increased,  as  must  always 
be  the  effect  of  producing  planes  that  may 
become  planes  of  cleavage  or  separation. 
Obviously  the  qualities  of  quenched  steel 


STEEL 


59 


will  depend  on  the  carbon  (carbide)  present 
in  solid  solution,  and  at  the  recalescence 
point  it  is  only  capable  of  retaining  carbide 
equal  to  0-89  per  cent,  carbon.  Above 
that  temperature  it  will  retain  more  in 
solution,  but  this,  as  shown,  separates  as 
the  temperature  falls  towards  the  critical 
point.  The  solid  solution  of  cementite  in 
quenched  steel  is  known  as  martensite.  It 
is  harder,  stronger,  more  elastic,  and  less 
ductile  in  proportion  to  the  carbon  it 
contains.  For  structural  purposes  the 
modification  of  the  cooling  rate  to  prevent 
lamination  of  the  pearlite  by  somewhat 
hastening  the  cooling,  so  as  to  produce  the 
sorbitic  structure,  is  sometimes  carried  out. 


Fig.  55. — Photo-micrograph  of  Laminated 
Pearlite  in  Steel  after  Prolonged 
Annealing 

Prolonged  annealing  and  slow  cooling  will 
produce  the  coarsest  lamination  and  the 
weakest  metal. 

Normal  air-cooled  metal  holds  a  position 
intermediate  between  the  two. 

With  regard  to  the  photo-micrographs 
here  shown,  Fig.  53  shows  hard  steel  (3  per 
cent,  carbon)  cooled  slowly;  the  borders 
of  cementite  (Fe3C)  surround  the  pearlite 
areas.  Fig.  54  is  a  low  magnification  in 
which  the  dark  areas  represent  pearlite. 
Fig.  55  (a  very  high  magnification)  shows 
laminated  pearlite  in  steel  after  prolonged 
annealing. 

In  carbon  steels  the  recalescence  tempera- 
ture varies  from  655°  C.  to  680°  C.  and  coin- 
cides with  the  temperature  at  which  the 
carbide  areas  assume  the  pearlitic  structure. 
Visible  redness  is  about  580°  C.  In  hardened 
steels  the  metal  is  in  a  condition  of  internal 
stress,  due  to  the  retention  of  the  carbide 
in  a  condition  that  is  not  normal  in  the  cold. 


The  raising  of  the  temperature  in  tempering 
operations  gives  a  certain  amount  of  mole- 
cular liberty,  and  to  some  extent  the  marten- 
sitic  condition  is  destroyed,  the  tendency 
being  for  the  carbide  to  assume  the  normal 
pearlitic  condition.  Softening  takes  place 
to  some  extent,  and  the  brittleness  is  re- 
duced. The  nearer  the  temperature  ap- 
proaches the  critical  point  and  the  greater 
the  latitude  given,  the  greater  is  the  change. 
At  the  critical  point  it  is  completed. 

With  mild  steels  there  are  other  tempera- 
tures at  which  the  rate  of  cooling  is  retarded. 
The  highest  point  is  at  880°  C.  Another 
occurs  at  735°,  and  the  third  about  680°. 
The  latter  coincides  with  the  critical  point 
already  noticed.  In  steels  free  or  nearly  free 
from  carbon  this  is  insignificant.  As  the 
carbon  content  increases,  the  upper  retarda- 
tions diminish  and  finally  disappear,  and 
the  whole  retardation  is  concentrated  at 
the  lower  point.  The  higher  critical  points 
are  ascribed  to  molecular  changes  in  the 
iron  itself.  These  cannot,  however,  be 
made  permanent  by  quenching,  and  in  the 
absence  of  carbon  their  effect  is  not  notice- 
able in  the  metal,  whether  cooled  rapidly  or 
slowly. 

In  alloy  steels  such  as  tungsten,  chrome, 
chrome  -  tungsten,  molybdenum,  nickel, 
manganese,  etc.,  the  added  metals  lower 
the  recalescence  point  and  bring  it  below 
ordinary  temperature.  Such  metals  in 
consequence  retain  their  hardness  and  are 
known  as  self -hardening  or  air-harden- 
ing steels.  They  can  be  heated  without 
becoming  softened,  and  hence  cutting 
speeds  which  develop  so  much  heat  as  to 
destroy  the  temper  of  ordinary  steel  tools 
can  be  employed  without  risk.  Hence 
the  term  rapid-cutting,  high-duty,  or  high- 
speed steels. 

METHODS   OF    MANUFACTURE 

Blister  or  cementation  steel  is  manu- 
factured by  heating  bars  of  nearly  pure 
iron  embedded  in  carbon  for  a  long  period. 
Such  metal  is  covered  with  blisters,  hence  the 
name.  It  is  usually  converted  into  shear 
steel  by  welding  together  a  number  of  such 
bars  to  secure  greater  uniformity. 

Cast  crucible  steel  is  made  by  melting 
blister  steel  in  crucibles  and  casting  into 
ingots.  Different  tempers  are  secured  by 
additions  of  pure  soft  iron,  carbon,  and  other 
ingredients  to  obtain  the  desired  product. 
The  principal  use  of  such  steel  is  for  cutting 


REINFORCED    CONCRETE 


instruments.     The  process  is  costly  and  the 
output  limited. 

Bessemer  steel  is  made  by  blowing  air 
through  molten  pig  iron  contained  in  a 
suitable  vessel.  The  metal  is  introduced  in 
the  molten  state.  The  carbon,  manganese, 
and  silicon  in  the  metal  are  removed  by 
oxidation,  and  sufficient  carbon  is  introduced 
by  adding  the  necessary  amount  of  a 
manganiferous  pig  iron — spiegeleisen — to  the 
metal.  The  operation  is  very  rapid,  a 
charge  of  several  tons  being  converted  in 
from  16  to  20  minutes.  The  metal  is  cast  into 
ingots  and  rolled.  In  the  basic  bessenier 
process,  the  lining  of  the  converter  is  made 
of  basic  material — calcined  dolomite — and 
this  permits  of  the  removal  of  the  phosphorus. 
The  operation  is  somewhat  longer.  In  both 
of  the  processes,  exact  control  of  the  carbon 
contents  is  difficult.  Owing  to  the  rapidity 
with  which  the  operation  must  be  con- 
ducted, there  is  no  time  for  its  determina- 
tion, and  it  is  impossible  to  hold  the  metal 
in  a  sufficiently  fluid  state  for  the  necessary 
length  of  time  to  enable  this  to  be  done,  as 
all  the  heat  required  is  generated  in  the 
vessel  by  the  oxidation  going  on  during  the 
passage  of  the  air  through  the  metal. 


Open  hearth  steel — usually  specified  for 
reinforced  concrete  work — is  made  from 
pig  iron  and  steel  scrap  in  a  gas-fired  furnace 
of  special  design,  known  as  a  regenerative 
or  open  hearth  gas-fired  furnace.  In  this 
furnace  the  materials  are  melted,  and  the 
carbon  and  other  elements  removed  by  the 
action  of  air  and  by  adding  iron  oxide  to  the 
charge.  The  heat  required  to  melt  and 
keep  the  metal  molten  is  supplied  by  the 
burning  gas.  The  time  occupied  extends 
over  some  hours,  dependent  on  the  weight 
of  the  charge  and  other  conditions.  Facilities 
are  thus  afforded  for  the  careful  control  of 
the  contents  of  carbon  and  other  elements, 
and  metal  of  a  definite  composition  can  be 
obtained.  In  the  basic  open  hearth,  a  basic 
lining  is  used,  and,  as  in  the  basic  bessemer 
process,  pig-iron  containing  phosporus  may 
be  used,  as  that  element  is  eliminated  during 
the  process. 

Basic  steel  is  the  term  applied  to  steel  made 
in  converters  or  furnaces  lined  with  basic 
materials,  such  as  calcined  dolomite,  magne- 
site,  and  chromite. 

Acid  steel  is  steel  made  in  converters  or 
furnaces  lined  with  siliceous  materials,  such 
as  silica  sand  and  bricks,  or  ganister. 


Stress  Simply  Explained 


EXPERIENCE  shows  that,  in  a  large  pro- 
portion of  cases,  the  failure  of  the  practical 
man,  and  even  of  the  student,  to  grasp  the 
essential  ideas  underlying  the  theory  of 
construction  in  reinforced  concrete  is  a 
direct  result  of  his  ignorance  of  the  simple 
mechanics  of  construction,  particularly  his 
lack  of  acquaintance  with  "  stress  " — a  term 
meaning  the  effect  produced  on  a  beam  by 
loading  it.  The  present  intention,  therefore, 
is  first  to  deal  with  the  fundamental  principles 
that  must  be  understood  by  anyone  desirous 
of  successfully  studying  the  constructional 
design  of  buildings,  and  then,  in  the  suc- 
ceeding chapter,  to  show  how  those  princi- 
ples are  applied  in  reinforced  concrete  con- 
struction. 

It  must  here  be  emphasised  that  it  is 
absolutely  essential  that  the  reasoning  upon 
which  the  various  formulae  are  based  should 
be  understood  if  these  formulae  are  to  be 
used  intelligently.  Students  frequently  fail 
to  grasp  the  theory  of  design  owing  to  the 
fact  that  they  do  not  start  with  a  sufficient 
knowledge  of  the  principles  of  force  and 
resistance.  In  many  instances  the  very 
expressions  that  are  used  are  imperfectly 
understood,  and  the  students  are  endeavour- 
ing to  obtain  the  value  for  something  of 
which  they  do  not  know  the  meaning,  and 
often  when  they  have  found  a  result  they 
do  not  realise  why  it  Was  necessary  to  find 
it.  To  the  general  reader  this  may  appear 
to  be  an  exaggeration  of  the  state  of  affairs, 
but  we  are  merely  expressing  views  that 
have  been  gathered  during  several  years  of 
tuition  to  building  students  of  all  kinds. 

FORMULA 

There  are  two  kinds  of  formulae  which  are 
met  with,  namely  "  rational  "  and  "  em- 
pirical." 

"  Rational  "  applies  to  formulas  that  have 
been  deduced  by  reasoning  alone,  such 
reasoning  being  based  upon  definite  axioms 
known  to  be  correct.  It  is  not  sufficient, 
therefore,  for  the  student  to  know  a  rational 
formula  ;  he  should  also  know  and  under- 
stand the  construction  or  reasoning  through 
which  the  formula  was  evolved. 


"  Empirical "  formulae  are  based  upon 
actual  experiments  through  which  certain 
values  have  been  obtained,  according  to  the 
material,  load,  and  disposition  under  the 
test ;  and,  as  such,  their  application  is 
usually  limited,  and  not  possible  for  cases 
which  do  not  agree  with  the  conditions  of 
the  test. 

Again,  it  will  be  necessary  to  use  certain 
factors  or  constants  derived  by  the  person 
responsible  for  the  test,  and  these  constants 
must  be  accepted  as  being  correct,  and 
become  merely  a  matter  of  memory  and  not 
of  reasoning.  There  are  some  cases,  how- 
ever, where  pure  theory  is  liable  to  give 
slightly  inaccurate  results,  owing  to  the 
peculiar  behaviour  of  some  materials  under 
a  varying  stress,  and  any  inconsistency  of  this 
kind  would  result  in  the  theory  not  agreeing 
with  the  practice.  An  excellent  instance  of 
the  difference  between  theory  and  practice 
is  that  explained  in  connection  with  columns 
and  struts  on  a  subsequent  page.  Wherever 
possible,  rational  formulae  are  to  be  pre- 
ferred, and  it  is  with  these  that  the  following 
notes  will  deal  in  the  first  instance. 

PRINCIPLE   OF    MOMENTS 

By  far  the  most  important  of  all  the  points 
to  be  grasped  is  that  of  the  "  principle  of 
moments,"  and  the  definition  of  this  princi- 
ple is  often  given  in  flowing  language  and 
repeated  by  the  student  without  a  proper 
understanding. 

A  "  moment "  really  means  an  amount, 
and  is  a  distinctive  term,  inasmuch  as  it 
means  not  only  the  actual  amount  of  a  force 
in  pounds,  hundredweights,  or  tons,  but  the 
actual  value  of  a  force  to  cause  stress  at 
any  point,  this  value  being  dependent  on  its 
amount,  direction,  and  disposition  in  relation 
to  other  forces  or  constructional  members. 
Thus,  "  taking  moments  round  a  certain 
point "  really  means  "  taking  values  or 
amounts  acting  at  that  point." 

Now,  the  "  principle  of  moments  "  is  that 
when  several  forces  are  acting  at  a  point 
and  equilibrium  is  produced,  then  all  those 
forces  that  would  tend  to  cause  movement 
in  one  direction  are  exactly  equalled  by  all 


61 


62 


REINFORCED    CONCRETE 


those  forces  that  would  tend  to  cause  move- 
ment in  another  direction. 

This  principle  may  seem  somewhat  obvious 
to  many  readers,  and  they  may  not  realise 
its  value  ;  but  upon  consideration  it  will 
be  seen  that  it  is  an  axiom  which  is  frequently 
used.  For  example,  in  the  case  of  a  beam 
that  carries  a  load  at  any  point,  it  is  obvious 
that  the  load  is  a  force  tending  to  cause 
movement  in  a  downward  direction,  and  if 
equilibrium  is  maintained,  then  there  must 
be  some  force  or  forces  at  work  which  are 
causing  or  exerting  an  upward  tendency ; 
this  force  is  supplied  by  the  reaction  or 
reactions,  which  together  must  equal  the 
load. 

LEVERS 

Before  investigating  the  method  of  ascer- 
taining the  value  of  the  reactions  with  any 
given  load,  it  will  be  advisable  to  give  a 
few  notes  on  the  three  orders  of  the  levers. 
If  these  levers  are  thoroughly  understood, 
matters  will  be  greatly  simplified,  as  most 
problems,  both  in  ordinary  construction  and 
in  reinforced  concrete  work,  can  be  practically 
reduced  to  a  question  of  leverage.  A  lever 
can  be  denned  as  a  member  which  turns  on 
a  point  known  as  the  fulcrum,  and  it  is  the 
relative  positions  of  the  forces  acting  on  the 
lever,  and  the  fulcrum,  which  give  us  the 
order  of  the  lever. 

"First  Order." — A  common  example  of 
a  lever  is  that  illustrated  in  Fig.  56,  which 
shows  a  crowbar  being  utilised  to  lift  up  a 
weight,  such  as  a  block  of  stone.  It  will  be 
seen  that  the  lever  is  tending  to  turn  on 
the  fulcrum  under  the  action  of  the  two 
forces  which  oppose  one  another  through  the 
medium  of  the  lever ;  this  is  an  example  of 
a  lever  of  the  first  order,  as  the  fulcrum  is 
between  the  weight  and  the  power.  Now 
the  value  of  the  power  and  the  weight  to 
cause  the  turning  of  the  lever  will  depend 
not  only  upon  their  actual  amount  in  pounds 
or  hundredweights,  but  also  upon  their  dis- 
tances from  the  fulcrum.  In  other  words, 
the  "  moment  "  of  the  power  at  the  fulcrum 
is  equal  to  its  amount  in  pounds  multiplied 
by  its  distance  to  the  fulcrum,  the  latter 
being  known  as  the  long  arm  of  the  lever ; 
this  is,  therefore,  20  Ib.  x  30  in.  =  600  in.-lb. 
The  weight  is  opposing  the  power,  and  if 
equilibrium  is  produced,  then  the  "  moment  " 
of  the  weight  at  the  fulcrum  must  also  equal 
600  in.-lb.,  and  this  moment  is  found  by 
multiplying  the  amount  of  the  weight  by  its 


distance  to  the  fulcrum.     Therefore  weight 
x    6  in.    =  600  in.-lb. 

Weight   =    f~     =  100  Ib. 
To  produce  equilibrium,  then,  we  must  have 


20  LB. 


Fig.   56. — Lever  of  First  Order 

a  weight  of  100  Ib.  and  a  power  of  20  Ib. 
if  the  arms  of  the  lever  are  6  in.  and  30  in. 
respectively.  Any  fraction  over  20  Ib;  in 
power  would  raise  the  weight  of  100  Ib.,  and 
thus  there  is  a  mechanical  advantage  or  a 
gain  in  the  value  of  the  power  by  the  intro- 
duction of  the  lever.  A  rule  can  now  be 
deduced  which  will  apply  to  all  levers  and 
enable  us  to  ascertain  the  weight  or  the 
power  required  to  produce  equilibrium  if 
the  value  of  one  force  is  known,  as  follows  : 
The  power  multiplied  by  its  distance  to  the 
fulcrum  is  equal  to  the  weight  multiplied 
by  its  distance  to  the  fulcrum. 

"  Second  Order."— In  the  first  order  it 
was  seen  that  the  fulcrum  was  situated  be- 
tween the  power  and  the  weight,  while  in 
Fig.  57  an  illustration  of  the  second  order  is 
given,  where  it  will  be  seen  that  the  weight 


POWER 


Rf5l5JANCE. 
OK.   WEIGHT 


Fig.   57.  —  Lever  of  Second  Order 


is  situated   between   the   fulcrum   and   the 
power. 

"Third  Order."—  In  this  (Fig.  58)  the 
power  is  between  the  weight  and  the  fulcrum. 
The  power  required  will  always  be  in  excess 
of  the  weight,  which  is  not  the  case  with 
the  other  orders.  The  student  is  advised  to 
work  out  examples  in  each  order  by  the  rule 


STRESS    SIMPLY    EXPLAINED 


given  above,  in  order  to  familiarise  himself 
with  the  principle  of  taking  moments  round 
the  fulcrum. 

REACTIONS 

A  reaction,  as  its  name  implies,  is  a  force 


POWER 


FULCRUM 


RL5I6TANCE 
OE.  WE.IOHJ 


Fig.   58.— Lever  of  Third  Order 

or  resistance  which  is  the  outcome  of  an 
action  or  force.  It  is  obvious  that,  when 
any  force  or  action  is  .introduced,  and 
equilibrium  is  still  maintained,  there  must 
be  a  reaction  or  reactions  which  are  equal 
to  the  initial  action,  but  opposite  in  tendency. 
No  difficulty  should  present  itself  in  ascer- 
taining the  values  of  these  reactions,  but  the 
student  should  fully  understand  the  method 
of  calculation  under  any  condition  of  load- 
ing before  seeking  to  understand  the  theory 
of  bending  moments. 

The  reactions  in  the  case  of  a  beam  can 
be  denned  as  the  passive  resistance  at  the 
supports  offered  by  the  strength  of  the 
materials  at  these  points  to  resist  crushing. 


/         "! 

'         A 

rt-t. 


-f-  CCNTBE  OP  GRAVITY 
/     Op  WALL 


CANJILCVER 


-18'- 


V 


Fig.   59. — Diagram  showing  how  a  Cantilever 
Tends  to  Move 


The  resistance  or  reaction  at  each  support 
Tvill  depend  on  the  amount  of  the  loading 
and  its  disposition  on  the  beam. 

Cantilever  Reactions. — In  the  case  of 
a  cantilever,  as  there  is  only  one  support, 
the  whole  of  the  reaction  must  be  supplied 
by  this  ;  at  the  same  time  there  is  a  tendency 
ior  the  cantilever  to  overturn,  due  to  the 


leverage  of  the  weight,  and  lift  up  the  work 
above  the  tailing-down  portion.  To  prevent 
this  overturning  movement  from  occurring, 
a  downward  reaction  or  resistance  will  be 
required  at  the  support,  and  its  value  must 
be  equal  to  or  greater  than  the  moment  of 
the  load.  This  resistance  will  be  supplied 
by  the  weight  of  the  brickwork  or  stonework 
over  the  tail  end  of  the  cantilever ;  or,  if 
this  is  insufficient,  anchor  bolts  may  be 
carried  down  into  the  lower  part  of  the  wall. 
In  Tig.  59  the  tendency  to  move  is  shown 
diagrammatically  by  the  dotted  lines,  and 
the  problem  will  be  quite  simple  if  the 
arrangement  is  considered  as  that  of  a  lever 
of  the  first  order.  The  overturning  moment 
at  the  point  F,  which  is  the  fulcrum,  will  be 
equal  to  the  weight  multiplied  by  its  dis- 
tance to  this  point,  which  equals  5  tons  x 


\    A     / 


Fig.   60.— Triangular   Portion  of  Wall    Lifted   by 
Movement  of  Cantilever 

12  ft.  =  60  ft.-tons.  The  weight  of  the  wall 
above  is  distributed  over  the  length  of 
18  in.,  and  for  the  purpose  of  calculating  its 
moment  at  F,  it  may  be  considered  as  acting 
at  its  centre  of  gravity,  namely,  9  in.  from 
the  fulcrum.  The  weight  of  the  wall  acting 
at  A  multiplied  by  9  in.  or  f  ft.,  must  there- 
fore equal  60  ft.-tons  =  expressed  as  an 
equation  : 

A  : 

A  : 


|  ft. 

|  ft. 

|  A 

A 

A 


=  W  x  I 

=5  tons  x  12  ft. 

=  60  ft.-tons 

=  60  ft.-tons  -*-  |  ft. 
=  80  tons 


The  weight  of  the  wall  over  the  end  of  the 
cantilever  must  therefore  exceed  80  tons  if 
the  latter  is  to  be  safe  against  overturning  ; 
or  some  other  form  of  anchorage  must  be 
provided  which  will  give  an  equivalent  of 
60  ft.-tons  at  F.  Owing  to  the  bonding  of  the 


64 


REINFORCED    CONCRETE 


material  of  which  the  wall  is  composed  it 
would  be  necessary  to  lift  all  the  work  inside 
the  dotted  lines  shown  in  Fig.  60,  and,  of 
course,  this  amount  would  be  considered  in 
the  calculations. 

Beam     Reactions. — Certain     variations 


load)  is  situated  between  the  resistance  and 
the  fulcrum.     Therefore  : 


W- 

^ 

t 

A 

W 

r8 

if'f\" 

Fig.  61. — Beam  Loaded  Eccentrically 

occur  in  the  case  of  beams,  as  there  are  two 
reactions  to  consider,  and  these  are  not 
necessarily  equal  to  one  another.  When  the 
beam  is  uniformly  loaded,  whether  the  loads 
are  concentrated  or  distributed,  the  reaction 
at  each  abutment  will  be  equal,  and  their 
combined  total  must  be  equal  to  the  sum 
of  the  loads  on  the  beam.  If  the  loading  is 
not  uniform,  then  the  reactions  will  vary, 
and  it  is  these  cases  that  will  be  dealt  with 
here. 

Let  a  beam  be  taken  with  a  span  of  15  ft. 
loaded  with  a  concentrated  weight  of  5  tons, 
situated  at  a  point  6  ft.  from  one  support, 
as  shown  in  Fig.  61.  A  greater  proportion 
of  the  load  will  be  carried  by  the  abutment 
A  than  by  the  abutment  B,  as  the  weight  is 
nearer  the  former,  and  the  reaction  will 
require  to  be  equal  to  the  amount  of  the  load 


Fig.   62. — Diagram  showing  how  Eccentrically 
Loaded  Beam  Tends  to  Rotate 

carried.  The  actual  amount  may  be  calcu- 
lated by  again  referring  to  the  principle  of 
the  levers.  In  Fig.  62  it  will  be  seen  that 
the  load  has  a  tendency  to  push  the  beam 
downward  and  exert  a  pressure  on  abutment 
A  by  rotating  on  the  abutment  B  at  point  F. 
This  is  an  example  of  a  lever  of  the  third 
order,  where  the  power  (supplied  by  the 


W  x  9  ft. 

5  tons  x  9  ft. 


=  resistance  at  A  x  15  ft. 
=  R1  x  15  ft, 
45  ft.  -tons 

15  ft. 
R1   =  3  tons 

The  reaction  at  A,  therefore,  is  3  tons,  and, 
if  the  sum  of  the  reactions  is  equal  to  the 
load,  the  reaction  at  B  =  5  tons  —  3  tons  = 
2  tons.  This  can  be  proved  by  considering 
the  beam  as  rotating  on  abutment  A  to  cause 
a  pressure  on  abutment  B,  as  indicated  in 
Fig.  63.  Then  : 

W  x  6  ft.   =  resistance  at  B  x  15  ft. 
5  tons  x  6  ft.    =  R2  x  15  ft. 
30  ft.-tons 


R2 


15  ft. 

2  tons, 

which  agrees  with  the  amount  above  stated. 


RESISTANCE' 
Fig.  63. — Beam  Rotating  on  the  other  Abutment 

A  rule  can  now  be  deduced  as  follows  : 
The  reaction  at  an  abutment  is  equal  to 
the  load  multiplied  by  its  distance  from 
the  opposite  abutment,  divided  by  the  whole 
span. 

If  more  than  one  load  is  carried,  then  the 
reaction  is  found  by  multiplying  each  load 
by  its  respective  distance  from  the  abutment, 
adding  the  amounts  thus  found,  and  divid- 
ing the  total  by  the  whole  span  ;  this  will 
be  made  clear  by  an  example.  The  example 
in  Fig.  64  shows  a  beam  which  carries  three 
different  concentrated  loads  ;  then  : 

R1  =  [  (3  tons  x  15  ft.)  +  (4  tons  x  10  ft.) 

+  (5  tons  x  6  ft.)  ]  -f-  18  ft. 
45  +40+30 

R     =    To 


18 


tons 


R2  = 

(Stons  x  3ft.)  +(4tons  x  8ft.)  +(5tons  x  12ft.) 
~ 


STRESS    SIMPLY    EXPLAINED 


9+32+60 


101 


18 
=  R2  =  5^4  tons 


R1  +  R2  must  equal  total  load  6T7¥  tons  + 
5  {*  tons  =  12  tons  —  total  load. 

Any  number  of  loads  may  be  dealt  with 
in  this  manner,  and  if  a  distributed  load  over 
part  of  the  length  only  has  to  be  considered, 
it  can  be  taken  as  acting  at  its  centre 
of  gravity  for  the  purpose  of  calculating 
the  reaction,  while  if  it  is  distributed 
uniformly  over  the  whole  length  it  is  ob- 
vious that  one  half  will  be  carried  by  each 
support. 

The  weight  of  the  beam  itself  has  purposely 
been  neglected  with  the  object  of  simplifying 
the  explanation,  but  when  it  is  required  to 
allow  for  this  it  should  be  dealt  with  as  a 
distributed  load  as  explained  above. 

Effective  Span.  —  The  beam  diagrams  in 


4TOI«>    S70N6 


\ 

i 

r                     i 

^ 

,     „ 

.  , 

" 

////> 

A  \ 

IQ'     nf 

l& 

i                                                                                       t 

r»2 

Fig.   64. — Beam  Carrying  Three  Concentrated 
Loads 

this  and  the  following  chapter  do  not  show 
any  bearing  on  the  support.  What  is 
actually  illustrated  is  the  effective  span,  this 
being  the  distance  between  the  centres  of 
the  bearing  surface,  where  the  pressure  is 
theoretically  considered  as  acting.  Fig.  65 
shows  how  the  portion  of  the  support  inside 
the  centre  of  pressure  and  the  portion  of 
the  beam  outside  the  same  point  are  neg- 
lected, these  portions  being  hatched.  The 
area  of  the  bearing  surface  will  be  proportion- 
ate to  the  amount  of  the  reaction,  and  will 
vary  with  the  material  of  which  the  support 
is  composed  ;  this,  of  course,  is  merely  a 
matter  of  dividing  the  total  load  on  the 
support  by  the  safe  load  per  unit  on  the 
material,  when  the  required  area  will  be 
given. 

BENDING  MOMENTS 

The  term  "  bending  "  needs  no  explana- 
tion ;   and  the  term  "  moment  "  has  already 
been  explained  ;   it  will  therefore  be  under- 
5 


stood  that  the  expression  "  bending 
moment "  refers  to  the  amount  or  value 
of  a  force  or  forces  to  cause  a  tendency  to 
bend.  This  value  will  depend  on  the 
actual  amount  of  the  force  and  on  its 
disposition. 


NTEE.OF  5EARIN6 


Fig.   65. — Diagram  Illustrating  Clear  and 
Effective  Spans 

Bending  Moment  at  Centre  of  Beam. 

— The  bending  moment  (B  M)  in  a  beam  or 
cantilever  is  caused  by  the  load  and  the 
reaction  opposing  one  another ;  and,  as  the 
beam  or  cantilever  is  the  agent  through 
which  this  opposition  is  allowed  to  act,  it  is 
called  upon  to  resist  a  certain  amount  of 
stress  due  to  the  bending  tendency. 

Assume  a  beam  to  carry  a  load  of  10  tons 
in  the  centre  of  its  length,  as  in  Fig.  66; 
The  load  will  set  into  force  an  action  which 
can  be  expressed  as  a  downward  action,  due 
to  the  force  of  gravity ;  in  order  to  produce 
equilibrium,  this  must  be  counteracted  by 
a  resistance  at  the  abutments  which  is  equal 
in  amount  and  opposite  in  direction.  There- 
fore, the  reactions  can  be  expressed  as 
upward  forces,  which  are  set  into  action  by 
the  passive  resistances  of  the  abutments. 
It  will  be  seen,  then,  that  these  actions  (the 
load  and  the  reactions)  exert  a  bending 
tendency  in  the  beam  through  which  they 
act  as  shown  (in  an  exaggerated  manner)  by 


i 

1 

f' 

; 

J 

| 

B'                                                       R2 

Fig.   66.— Centrally-loaded  Beam 

Fig.  67,  in  which  the  beam  has  a  tendency 
to  turn  on  the  point  F. 

Now,  when  two  parallel  forces  are  acting 
in  opposite  directions  and  opposing  one 
another,  they  are  said  to  form  a  couple,  and 
their  greatest  value  to  cause  stress  is  the 
amount  of  one  force  multiplied  by  the 


66 


REINFORCED    CONCRETE 


distance  between  them.     In  the  instance  just 

W 

given,  the  greatest  value  is  -^  multiplied  by 

I 

o,  where  W  equals  the  total  weight  and  I 

equals  the  total  span.     It  is  obvious  that 


x    „-  which 


Fig.  67.  —  Bending  Tendency  on  Beam 

half  the  weight  is  taken  by  each  abutment, 
and  therefore  the  opposition  from  each  end 
of  the  beam  can  only  be  due  to  this  amount. 
This  is  explained  by  Fig.  67.  Hence  the 
formula  for  finding  the  greatest  bending 
moment  in  the  case  of  a  beam  with  a  central 

W 

concentrated  load  is  B  M   =  -~ 

WZ 
equals  B  M  =   -r~> 

W  I 

This  formula   -j-   expresses  the  greatest 

bending  moment  in  the  case  of  a  supported 
beam  carrying  a  concentrated  central  load, 
and  this  greatest  bending  moment  will  be 
found  to  occur  at  the  centre  of  the  span, 
where  moments  have  just  been  taken. 

Bending  Moment  at  any  Point  in 
Beam.  —  Now  consider  the  method  of  ascer- 
taining the  bending  tendency  at  any  point 
other  than  the  centre.  By  taking  any 
particular  example  and  calculating  the  B.M. 
at  various  points,  it  will  be  seen  how  the 
variation  becomes  uniform  and  diminishes 
from  the  centre  of  the  beam  to  the  abut- 
ment, where  it  becomes  nil.  This  uniform 
variation  will  only  be  found  to  occur  in  the 
case  of  beams  and  cantilevers  carrying  con- 
centrated loads,  and  it  will  be  shown  later 
how  the  variation  occurs  in  the  case  of  dis- 
tributed loads.  Assume  a  beam  to  carry  a 
concentrated  load  of  10  tons  at  the  centre 
of  the  span  which  is  10  ft.,  as  before,  then, 

Wl 


according  to  the  formula  B  M  = 


the 


is  due  to  the  reaction  multiplied  by  its  dis- 
tance to  the  point  at  which  the  moments  are 
taken  ;  and  on  applying  this  principle  it  will 
be  found  quite  easy  to  ascertain  the  B.M. 
at  any  point.  Assume  the  same  beam  with 
the  concentrated  load  of  10  tons  at  the 
centre,  and  find  the  B.M.  at  a  point  3  ft. 
from  one  support  (see  Fig.  68).  Now,  the 
reaction  at  each  support  will  be  equal  to 
5  tons,  therefore,  it  is  only  necessary  to 
multiply  the  reaction  at  A  by  its  distance  to 
the  point  at  which  the  B.M.  is  required, 
namely,  3  ft.  Hence,  the  B.M.  =  5  tons 
x  3  ft.  =  15  ft.-tons. 

Again,  the  B.M.  at  a  point  2  ft.  from  the 
support  =  5  tons  x  2  ft.  =  10  ft.-tons,  thus 
showing  how  the  B.M.  diminishes  towards 
the  abutment,  and  as  the  reaction  is  a  con- 
stant figure  in  the  calculations  for  the  B.M. 
at  any  point,  and  the  only  factor  that  varies 
is  the  distance  of  the  point  from  the  support, 
it  can  be  said  that  the  B.M.  at  any  point 
varies  directly  as  its  distance  from  the  support. 

If  the  B.M.  at  the  centre  is  set  up  to  scale 
over  the  beam,  as  shown  in  Fig.  69,  and  the 
span  of  the  beam  itself  is  drawn  to  scale, 
then  the  B.M.  at  all  points  will  be  expressed 
graphically  by  this  diagram,  and  by  scaling 
the  vertical  line  at  any  intermediate  point 
the  B.M.  at  that  point  can  be  ascertained. 

This  explanation  has  now  deduced  the 
following  rule  : 

The  bending  moment  at  any  point  in  a 
beam  with  a  concentrated  load  equals  the 
reaction  at  the  abutment  multiplied  by  its 
distance  to  that  point. 


r~ 

*! 

i 

B1. 

It!? 

f.                 T  '  s*  "                  — 

/ 
\ 

5  TO' 

5"-0           >• 

ir>' 

n" 

i                           i 

5  TOto                                e*= 

.    '  10  x  10 

bending  moment  = j —    =  25  ft.-tons. 

As  already  explained,  the  bending  tendency 


Fig.   68. — Finding  Bending  Moment  of 
Beam 

Weight  between  Reaction  and  Point 
of  Calculation. — If  the  weight  is  situated 
between  the  reaction  selected  and  the  point 
round  which  moments  are  taken,  then  allow- 
ance must  be  made  for  the  moment  of 
the  weight  at  this  point. 

This  can  be  explained  in  the  following 
manner :  Again  assume  the  same  condition 
of  loading  and  span  and  also  that  the  B.M. 


STRESS    SIMPLY    EXPLAINED 


67 


at  a  point  3  ft.  from  abutment  B  is  required 
to  be  calculated  from  abutment  A  (see  Fig. 
70).  It  will  be  seen  that  W  is  situated 
between  the  abutment  selected  and  the 
point  at  which  the  B.M.  is  to  be  found. 
Then  W  is  situated  on  the  arm  of  the  lever 


TO6CALE 


Fig.  69.— Bending  Moment  Set  up  to 

Scale 


with  two  loads  concentrated  at  different 
points,  and  we  will  assume  that  it  is  neces- 
sary to  ascertain  the  B.M.  at  the  centre  of 
the  beam.  Then  it  will  be  necessary,  which- 
ever abutment  is  selected  as  the  one  to  work 
from,  to  calculate  with  a  Weight  between  the 


f 
«.            r'-o'1- 

(OTOtli    ./ 

UPWARD  LEVC.RAGE- 
op  ts1 

f              , 

m 

•f  //I  '  n" 

*-2'o;-l 

—  S'O'^-, 

LL 

V 

i 

1 
5  TOS5                            DOWNWARD  LtVtR 

o^  w 

Fig.  71. — Lever  Arms  of  Reaction  and 
Load 


working  from  A,  and  the  downward  effect 
of  W  must  be  subtracted  from  the  upward 
effect  of  R1  at  A  at  the  point  where  the 
B.M.  is  to  be  found. 

Then  BMatF  — R1  x  upward  leverage 
-  W  x    downward  leverage,  as  shown  in 
Fig.  71. 

From  this  equation  it  will  be  seen  that 
the  B.M.  at  F  actually  equals  (5  tons  x  7  ft.) 
-(10  tons  x  2ft.)  =  35  ft.-tons -20  ft.-tons. 
=  15  ft. -tons — the  same  result  as  that 
obtained  in  the  previous  calculation  for  the 
B.M.  at  a  point  3  ft.  from  one  support  with 
similar  load  and  span. 

The  principle  previously  explained  must 
be  thoroughly  understood,  because  it  will 
be  seen  that  in  the  case  of  a  beam  which 


3-0'- 


•S'C" 


57010 


Fig.   70. — Finding  Bending  Moment  of 
Beam 

carries  two  or  more  concentrated  loads  at 
different  points  it  will  be  quite  impossible 
to  determine  the  B.M.  at  any  point  between 
two  of  the  loads  without  considering  the  load 
which  is  situated  on  the  arm  of  the  lever. 
Take  an  actual  example  showing  these  con- 
ditions. 

The  diagram  (Fig.  72)  illustrates  a  beam 


reaction  and  the  point  at  which  moments 
are  to  be  taken. 

Working  from  abutment  A,  then 
2  x  12  +  4  x  6 
~ 


Ri   =  33  tons. 
B  M  at  centre  =  R1   x 
-  W1  x 


distance  to  centre 
its    distance    to 

centre. 

B  M   =  3f-  tons  x  7  ft.  -  2  tons  x  5  ft. 
B  M   =  24  ft.-tons  -  10  ft.-tons. 
B  M   =  14  ft.-tons. 

The  following  rule  can  now  be  deduced  : 
The  bending  moment  at  any  point  in  a  beam 
which  carries  one  or  more  concentrated 
loads  can  be  found  by  multiplying  either 


W  -  2  T&N6 


Z-  4  TON5 


, 

^ 

»2.'o"-4«  —  6'-o  »|«  —  6  -o  *• 

1 

K'  n'                     > 

'^ 

t 

Fig.  72.- 


-Beam  with  Two  Loads  Concentrated 
at  Different  Points 


reaction  by  its  distance  to  the  point  at  which 
the  B.M.  is  to  be  found,  but  if  any  weights 
situated  between  the  reaction  selected 


are 


and  the  point  at  which  the  moment  is  to  be 
found,  then  a  deduction  must  be  made  equal 
to  the  sum  of  the  weights  multiplied  by 
their  respective  distances  to  this  point. 
This  rule  can  be  applied  to  any  number 


68 


REINFORCED    CONCRETE 


of  loads,  and  once  this  principle  is  thoroughly 
understood  no  difficulty  will  be  found  with 
a  beam  carrying  any  number  of  concentrated 
loads,  however  the  latter  may  be  situated. 
Bending  Moment  in  Cantilever. — The 
mathematical  method  of  determining  the 


W 


Fig.   73. — Cantilever  with  Concentrated  Load 
at  Outer  End 

B.M.  with  cantilevers  carrying  concentrated 
loads  will  now  be  considered. 

Owing  to  the  fact  that  only  one  support 
has  to  be  dealt  with,  the  calculations  are 
much  simplified,  as  it  will  be  obvious  that 
the  reaction  is  always  equal  to  the  load ; 
and  no  method  is  required  to  ascertain  this 
as  in  the  case  of  a  beam. 

The  most  simple  case  will  be  found  to  be 
that  of  a  cantilever  that  carries  a  single 
concentrated  load  at  its  extreme  outer  end, 
as  in  Fig.  73.  The  greatest  bending  tendency 
due  to  this  weight  will  be  found  to  occur  at 
the  support.  This  bending  tendency  is  due 
to  the  weight  pressing  down  on  the  cantilever, 
and  tending  to  cause  failure  by  turning  on 
the  point  F  as  shown  in  Figs.  74  and  75. 

Now  if  the  moments  round  this  point  F 


Fig.   74. — Failure  of  Cantilever  due  to 
Tension 

are  taken,  the  moment  of  the  weight  is  equal 
to  its  value  in  Ibs.,  cwts.,  or  tons  x  by 
its  distance  or  leverage  to  this  point.  This 
moment,  then,  is  equal  to  W  x  I,  where  W 
equals  the  weight  and  I  the  length  of  the 
cantilever. 

Hence,  the  formula  for  finding  the  greatest 
bending  moment  in  the  case  of  a  cantilever 


which  carries  an  end  load  is  B  M  =  W  I. 
The  following  is  an  example.  A  canti- 
lever with  a  projection  of  10  ft.  carries  an 
end  load  of  5  tons.  Then  B  M  =  W  I  or 
B  M  =  5  x  10  -  50  ft.-tons. 

To  ascertain  the  B.M.  at  any  intermediate 


Fig.  75. — Failure  of  Cantilever  due  to 
Compression 

point  it  is  only  necessary  to  apply  the  same 
principle — namely,  multiply  the  weight  by 
its  leverage  to  the  point  at  which  the  moment 
is  to  be  found.  Take  the  same  \\eight  and 
projection  as  in  the  last  example,  and  assume 
that  it  is  necessary  to  calculate  the  B.M.  at 
a  point  5  ft.  from  the  extreme  end.  Then 
it  will  be  seen  that  the  bending  tendency  at 
this  point  is  less  than  that  at  the  support, 
as,  although  the  weight  remains  the  same, 
the  leverage  has  diminished,  and  is  now  only 
5  ft.  instead  of  10  ft.,  as  Fig.  76.  The 
B.M.  will  now  equal  5  tons  x  5  ft.  =  25 
ft.-tons. 

A  rule  may  now  be  deduced  as  follows  : 
The  bending  moment  at  any  point  in  a 
cantilever  which  carries  a  single  load  at  its. 
outer  end  can  be  found  at  any  point  by 


-  5 


I'iO-O 


Fig.   76. — Bending  Moment  in  Cantilever 

multiplying  the  load  by  its  distance  to  that 
point. 

It  will  be  seen  that  one  factor  in  the  calu- 
lations  always  remains  the  same — namely, 
the  weight,  and  it  is  only  the  distance  or 
leverage  that  varies.  It  has  been  shown 
that  the  greatest  B.M.  occurs  at  the  support,, 
and  it  will  be  obvious  that  there  is  no  bend- 


STRESS    SIMPLY    EXPLAINED 


69 


ing  tendency  at  the  extreme  outer  end,  as 
the  weight  at  this  point  will  have  no  lever- 
age. Therefore,  the  greatest  B.M.  will  occur 
at  the  support,  and  it  will  diminish  from 
this  point  to  the  outer  end,  where  it  is  nil. 
It  is  also  known  that  the  diminution  will  be 
uniform,  as  it  is  dependent  only  on  the  dis- 
tance from  the  support.  Therefore,  if  the 
B.M.  at  the  support  is  set  up  to  scale  at 
this  point  and  a  straight  line  is  drawn 
through  to  the  outer  end,  a  diagram  will  be 
obtained  which  will  represent  the  B.M.  to 
scale  at  all  points  in  the  cantilever  due  to 
the  end  load,  as  Fig.  77. 

Bending  Moment  in  Cantilever  carry- 
ing more  than  one  Concentrated  Load. 
— In  the  case  of  a  cantilever  which  carries 
two  or  more  concentrated  loads  at  different 


Fig.   77. — Bending  Moment  of  Cantilever  Set 
out  to  Scale 

points,  it  is  only  necessary  to  apply  the 
same  principle  of  multiplying  the  loads  by 
their  respective  leverages  at  any  particular 
point,  and  the  sum  of  their  values  will  give 
the  bending  moment.  There  are  one  or 
two  points,  however,  to  be  borne  in  mind, 
and  the  chief  of  these  is  the  fact  that  a  load 
does  not  cause  a  bending  tendency  at  any 
point  which  is  not  situated  between  this 
load  and  the  support. 

An  illustration  will  render  this  quite  clear. 
In  Fig.  78  the  load  represented  by  W  will 
only  cause  a  bending  tendency  in  that  part 
of  the  cantilever  marked  A,  and  no  bending 
tendency  whatever  in  the  part  marked  B, 
which  is  not  between  the  load  and  the  sup- 
port, and  this  is  obvious,  as  the  load  will 
travel  inwards  to  the  support  over  the  por- 
tion A  to  meet  the  reaction,  and  there  will 
be  no  tendency  to  travel  outwards  over  B, 
as  no  resistance  is  offered  by  the  outer  end. 

Take  an  example  with  two  concentrated 


loads  and  calculate  the  B.M.  at  different 
points.  This  will  show  how  the  moments 
are  to  be  found. 

The  diagram,  Fig.  79,  shows  two  loads  of 
2  tons  and  4  tons  respectively.  The  B.M. 
at  the  support  will  be  due  to  each  weight 


Fig.    78. — How  Centrally-loaded  Cantilever 
Tends  to  Bend 

multiplied  .  by  its  respective  leverage,  and 
these  must  be  added,  as  they  are  not  opposing 
forces,  but  forces  acting  together. 

Then  B  M  at  support  =  (W1  x  6  ft.)  + 
(W2  x  9  ft.)  =  2  tons  x  6  ft.  +  4  tons  x 
9  ft.  =  12  +  36. 
L  B  M  =  48  ft.-tons. 

As  a  weight  will  not  cause  any  bending 
tendency  at  a  point  which  is  not  between 
the  weight  and  the  support,  W1  will  not 
cause  any  B.M.  between  W1  and  W2,  and 
the  only  tendency  will  be  due  to  W2,  as 
Fig.  80. 

B  M  under  W1  then  equals  W2  x  3  ft.  = 
4  tons  x  3  ft.  =12  ft.-tons. 

Bending  Moment  in  Beam  carrying 
Distributed  Load.  —  Concentrated  loads 
only  have  been  dealt  with  up  to  the  present, 
and  although  the  same  principles  apply  in 
the  case  of  distributed  loads  a  few  brief 


- 2 TONS  WMTON5 

4— 


6-0'- 


Fig.   79. — Cantilever  with  Two  Concentrated 
Loads 


notes  should  be  of  value,  more  especially  as 
distributed  loads  are  more  generally  met 
with  in  practice.  A  common  case  that  has 
to  be  calculated  is  that  of  a  beam  carrying 
both  concentrated  and  distributed  loads,  and 
an  example  of  this  nature  presents  great 
difficulty  to  one  who  is  not  well  versed  in 
structural  design. 


REINFORCED    CONCRETE 


As  a  first  example,  consider  a  beam  which 
carries  a  uniformly  distributed  load  over  its 
whole  length,  as  shown  in  Fig.  81.  Then  the 
total  load  will  be  the  length  multiplied  by 
the  weight  per  foot  run.  This  equals  10  ft. 
x  1  ton  per  foot  =  10  tons.  The  reactions 


The  greatest  bending  moment  then  equals 
W         I [         W        I 
"2"     :    2  '      2          I 
Wj        Vfl        Wj 

~T    ~    IT    =   "8" 


i          V 

W 
2 

1           1 

f         W 

J 

;         4 

t          J 

1 

?i 

—  hiq 

7 

'//" 

P 

\              z           ^~            1 

Fig.   80. — Bending  Tendency    in    Cantilever  with 
Two  Concentrated  Loads 


will  obviously  be  equal,  and  must  together 
equal  the  total  weight,  which  may  be 
expressed  as  W;  Then  R1  and  R2  each  = 

W 

-o-.     The  bending  moment  at  any  point  is 

equal  to  the  reaction  multiplied  by  its  dis- 
tance to  that  point,  minus  any  weight 
situated  between  the  reaction  and  the  point 
at  which  the  bending  moment  is  to  be  found 
multiplied  by  its  distance  to  the  same 
point.  The  greatest  bending  moment  will 
be  at  the  centre  of  the  span,  and  upon 
reference  to  Fig.  82  it  can  be  seen  what 
values  have  to  be  taken. 

W 

There  R1  equals  -^t  and  this  acts  upwards 

I 
with  a  leverage  of  ~  at  the  point  F,  which  is 

the  centre  of  the  span.     There  is,  however, 


/ 


LOAD  -  I  ion  pec  POOJ 


t-\o'-o- 


p'  I 

Fig.  81. — Beam  with  Uniform  Distributed 
Load 


Fig.  82.— Distributed    Load  Acting  through 
Centre  of  Gravity 


This  is  the  formula  for  the  greatest  bending 
moment  in  the  case  of  a  beam  which  carries 
a  uniformly  distributed  load  over  its  whole 
length.  To  work  out  the  example  in  Fig.  81 
it  is  only  necessary  to  apply  the  formula  as 
follows  : 

Cr  13  M  =     Q      =  ~  •    =  ~TT~    =  12'<> 

o  o  o 

ft.-tons. 

Let  it  be  now  assumed  that  the  bending 
moment  is  required  to  be  calculated  at  a 
point  3  ft.  from  abutment  A. 

Then  the  length  of  the  lever  arm  is  reduced 
to  3  ft.,  and  in  addition  the  load  acting 
downward  is  reduced  to  3  tons  with  a  lever- 
age to  the  point  F  of  1  ft.  6  in.,  as  in  Fig.  83. 
The  bending  moment  equals  : 

3  JONS 


Fig.  83. — Bending  Moment  at  Intermediate 
Point 


a  load  on   the   lever   arm  which   is   acting 
downward,    and    this    load    is    equal     to 

-Q-,  and  being  uniformly  distributed,  it  can 

be   considered   as   acting   at  it  3   centre   of 
gravity,  which  will  be  situated  at  a  distance 

equal  to  -r  from  point  F. 


(5  tons  x  3  ft.)  -  (3  tons  x  1-5  ft.) 
=  15  ft.-tons  —  4'5  ft.-tons 
=  10-5  ft.-tons. 

Although  the  reaction,  which  is  one  of  the 
factors  in  the  calculations  for  any  inter- 
mediate point,  remains  the  same,  the  lever- 
age diminishes  towards  the  support,  and  in 
addition  to  this,  the  weight  acting  down- 


STRESS    SIMPLY    EXPLAINED 


ward,  together  with  its  leverage,  diminishes 
as  the  lever  arm  of  the  reaction  becomes 
less.  The  diminution  of  the  bending  moment 
from  the  centre  of  the  span  to  the  support 
is  therefore  not  regular,  as  in  the  case  of  a 
central  concentrated  load,  but  actually 
varies  in  a  compound  ratio.  If  the  bending 
moment  at  the  centre  be  set  up  to  scale 
over  the  beam,  then  the  parabolic  curve, 
as  shown  in  Fig.  84,  will  give  the  bending 
moment  at  all  intermediate  points.  With 
regard  to  a  combination  of  distributed 
and  concentrated  loading,  an  example  is 
given  in  Fig.  85,  and,  if  this  is  explained, 
no  difficulty  should  be  experienced  with 
other  examples,  as  the  principles  are 


Fig.   84. — Bending  Moment  on  Beam  Set  up 
to  Scale 


always  the    same,    and    the    procedure    is 
quite  simple. 

F,irst  calculate  the  reactions  : 

K1  = 

6  tons  (distributed  load)  x  9  ft.  +  8  tons  x  4  ft. 
12  ft. 


E1   = 


54  +32 
12 


--S- 


under  the  load  of  8  tons  is  to  be  found  by 
working  from  abutment  A,  then  : 
B  M  =  (7J  tons  x  8  ft.)  -  (6  tons  x  5  ft.) 
B  M  =  57J  ft.-tons  -  30  ft.-tons  =  27£  ft.- 
tons.     This  can  be  checked  by  working  from 
abutment  B.     Then  : 

B  M  =  6    tons  x  4  ft.  =  27    ft.-tons. 


I  TON   PER  fOOT 


Fig.   85. — Beam  with  Concentrated  and 
Distributed  Loading 

The  bending  moment  at  any  other  point  can 
be  found  in  a  similar  manner,  and,  if  neces- 
sary, a  diagram  set  up  showing  the  value  at 
all  parts  of  the  beam. 

Bending  Moment  in  Cantilever  Carry- 
ing Distributed  Load. — In  the  case  of  a 
cantilever  carrying  a  uniformly  distributed 
load  over  its  whole  length,  the  greatest 
bending  moment  occurs  at  the  support,  and 
the  method  of  finding  its  value  is  very  simple.- 

The  portion  of  the  weight  at  the  extreme 
outer  end  of  the  cantilever  will  have  a  lever- 
age equal  to  I,  while  the  portion  of  the  weight 
at  the  extreme  inner  end  will  have  a  leverage 
equal  to  nil.  The  mean  leverage  may  there- 


Ws 

§ 

i 

i 

/  1  TON  PE2  fOOJ 

J                    > 

i 

m 

~-  4-z'-6'-H 

5-0"  »• 

Fig.  86. — Cantilever  Carrying  Distributed 
Load 


R2  = 


R2  = 


8  tons   x   8  ft.  +  6  tons   x   3  ft.      fore  be  assumed  to  be  ^   which   represents 

12  ft.  distance  from  the  support  to  the  centre  of 

64  +  18  gravity  of  the  weight.     The  formula  for  the 

~~12  greatest  bending  moment  then  becomes  : 

W   x    2    =  ~2~ 
In  the  exampls  in  Fig.  86,  the  greatest 


82 


=  jTj    =  6£  tons. 
Assume   that   the   bending   moment   at   F 


REINFORCED    CONCRETE 


bending    moment    is    equal   to   the  weight 

I 
of  5  tons  multiplied  by  v>,  which  is  2  ft.  6  in.  ; 

this  then  gives  a  moment  of  5   x  2  ft.  6  in. 
=  12J  ft.-tons. 


Fig.   87. — Finding  Bending  Moment  at  Inter- 
mediate Point  in  Cantilever 

In  the  case  of  the  bending  moment  at  any 
intermediate  point,  it  is  only  necessary  to 
consider  that  portion  of  the  weight  situated 
between  the  point  and  the  extreme  outer 
end.  As  an  example,  assume  that  the  bend- 
ing moment  is  to  be  calculated  at  the  point 
F  in  the  cantilever  shown  in  Fig.  87.  Then 
the  weight  to  be  considered  is  3  tons,  and 


Fig.  88. — Bending  Moment  in  Cantilever  Set 
out  to  Scale 

acting  at  its  centre  of  gravity  it  will  have  a 
leverage  of  1  ft.  6  in.  to  the  point  F.  The 
bending  moment  =  3  tons  x  1  ft.  6  in.  = 
4J  ft.-tons.  The  moment  varies  in  a  com- 
pound ratio,  as  in  the  case  of  a  beam,  owing 
to  the  weight  diminishing,  because  the  point 
at  which  the  moment  is  found  is  moved  out- 


ward from  the  support,   and  at  the  same 
time  the  lever  arm  also  decreases. 

If  the  bending  moment  at  the  support  is 
set  up  to  scale  as  illustrated  in  Fig.  88,  a 
diagram  can  be  drawn  to  show  the  bending 


I  TON  RLE  FOOT 

3  TON5 


TONi 


IZ'-O" 


Fig.   89. — Cantilever  with  Combined  Distributed 
and  Concentrated  Loading 

moment  at  all  points  in  the  cantilever  due 
to  a  distributed  load. 

Combined  Distributed  and  Concen- 
trated Loading  on  Cantilever. — With  re- 
gard to  a  combination  of  distributed  and 
concentrated  loading  on  a  cantilever,  an  ex- 
ample is  given  in  Fig.  89,  and  the  greatest 
bending  moment  will  be  calculated  as  an 
illustration  of  the  method  to  be  employed. 
It  will  be  seen  that  three  loads  are  carried, 
namely,  a  load  of  3  tons  5  ft.  from  the  sup- 
port, a  load  of  5  tons  at  the  extreme  outer 
end,  which  is  12  ft.  from  the  support,  and  a 
distributed  load  of  12  tons,  which  can  be 
considered  as  acting  with  a  leverage  of  6  ft. 
The  greatest  bending  moment  will  therefore 
be:— 

(3  tons  x  5  ft.)  +  (5  tons  x  12  ft.)  +  (12  tons 

x  6  ft.) 

=  15  ft.-tons  +  60  ft.-tons  +  72  ft.-tons 
=  147  ft.-tons. 

The  bending  moment  at  any  other  point 
can  be  found  by  considering  only  that  por- 
tion of  the  loading  which  is  situated  between 
the  point  and  the  outer  end  of  the  cantilever. 

MOMENT  OF  INERTIA 

The  moment  of  inertia  is  extremely  import- 
ant in  all  cases  of  advanced  calculations.  The 
expression  is  used  to  define  the  relative 
values  of  sections  of  different  shapes.  From 
the  moment  of  inertia,  the  section  modulus 
and  moment  of  resistance  (two  terms  that 
will  be  explained  later)  can  be  found. 

Explanation  of  Moment  of  Inertia.— 
The  moment  of  inertia  of  a  section  is  an 
indication  of  the  strength  of  that  section. 
It  is  a  constant  value,  and  depends  on 
the  shape  of  the  section.  The  area  alone 


STRESS    SIMPLY    EXPLAINED 


73 


is  no  indication  of  the  strength  of  a  section 
from  a  structural  point  of  view.  Two 
sections  of  the  same  area  do  not  necessarily 
have  the  same  power  to  resist  stress.  For 
example,  place  a  piece  of  timber,  9  in.  by 
2  in.  in  section,  as  a  beam  over  a  certain 
span — say  10  ft. — it  will  have  a  certain 
amount  of  power  to  resist  stress,  or,  in  other 
words,  a  certain  amount  of  power  to  remain 
"  inert,"  which  power  can  be  expressed  as  its 
*'  moment  of  inertia."  The  timber  will  have 
two  values,  according  to  the  Way  in  which 
it  is  placed.  In  the  form  of  a  beam  9  in. 
deep  and  2  in.  wide,  it  \vill  be  much  stronger 
as  a  weight-carrying  member  than  as  a  beam 
2  in.  deep  and  9  in.  wide.  This  can  be 
proved  by  a  practical  demonstration  quite 
easily,  or  by  the  theory  of  the  moment  of 
resistance  for  a  rectangular  beam,  to  be 
given  later. 

The  reason  for  this  difference  in  strength 
with  the  same  section  is  entirely  due  to  the 
disposition  of  the  fibres  in  relation  to  the 
neutral  axis  of  the  section,  which  is  different 
in  each  case.  (See  page  3  for  an  explana- 
tion of  what  is  meant  by  the  term  "  neutral 
axis.")  In  the  first  instance,  the  beam  was 
9  in.  deep,  and  therefore  some  of  the  fibres 
were  situated  at  a  distance  of  4i  in.  from  the 
neutral  axis,  whereas  in  the  second  instance 
the  beam  was  only  2  in.  deep,  and  therefore 
the  extreme  fibres  were  only  1  in.  from  the 
neutral  axis.  Now  the  stress  in  a  beam  is 
nil  at  the  neutral  axis,  and  it  increases  in 
intensity  as  it  gets  farther  away  from  the 
neutral  axis.  This  fact  allows  the  fibres  that 
are  at  a  distance  from  the  neutral  axis  to  be 
utilised  to  their  fullest  capacity,  whereas 
those  adjoining  the  neutral  axis  cannot  be 
called  upon  to  exert  their  greatest  resistance. 
Again,  it  will  be  shown  later  that  the  lever 
arm  of  the  tensional  and  compressional  areas 
is  increased  as  the  depth  is  increased,  and 
this  greatly  increases  the  value  of  the  resist- 
ance offered. 

These  remarks  should  suffice  for  the  pre- 
sent to  show  that  the  area  of  a  section  alone 
is  no  criterion  as  to  the  strength  of  the 
member,  and  that  the  same  section  will 
possess  two  different  capacities  according  to 
the  way  in  which  it  is  placed. 

It  will  now  be  understood  why  the  two 
values  for  the  moment  of  inertia  of  a  section 
are  known  as  the  "  least  moment  of  inertia  " 
and  the  "  greatest  moment  of  inertia," 
according  to  the  neutral  axis  around  which 
the  "  moment "  has  been  taken.  In  the 


case  of  a  section  which  is  square  or  circular 
there  will  only  be  one  value  for  the  moment 
of  inertia,  as  the  axis  will  be  at  the  same 
distance  from  the  extreme  fibres  in  both 
cases,  except  in  the  case  of  the  square 
section  being  placed  on  one  edge,  which  is 
so  unusual  in  structural  work  that  it  need 
not  be  considered  for  the  present.  The 
moment  of  inertia  in  value  \vill  be  dependent, 
in  every  case,  on  the  area  of  the  section,  and 
the  disposition  of  that  area  in  relation  to  the 
neutral  axis  of  the  section. 

Calculating  Moment  of  Inertia. — The 
method  of  finding  the  moment  of  inertia 
of  a  plane  figure  is  based  on  the  following 
principle.  The  section  or  plane  surface  is 


Fig.  90. — Calculating  Moment  of  Inertia  of 
Simple  Rectangular  Section 


imagined  to  be  divided  into  an  infinite  num- 
ber of  thin  layers,  and  the  area  of  each  layer 
is  multiplied  by  the  square  of  the  distance 
between  the  centre  of  gravity  of  the  layer 
and  the  axis  ;  the  sum  of  all  these  products 
is  the  moment  of  inertia  of  the  plane  surface 
with  respect  to  that  axis. 

An  attempt  should  now  be  made  to  calcu- 
late the  moment  of  inertia  of  a  simple  rect- 
angular section  as  given  in  Fig.  90.  Assume 
that  the  section  is  12  in.  deep  and  6  in.  wide, 
and  the  moment  of  inertia  is  to  be  taken 
about  the  axis  x — x.  Also  assume  that 
there  are  three  layers  on  each  side  of  the 
axis  2  in.  thick.  Then  the  moment  of 
inertia  will  equal 
[(6  x  2  x  52)  +  (6  x  2  x  32)  +  (6  x  2  x  I2)] 

x  2  =  [300  +  108  +  12]  x  2  =  420 

x   2   =  840. 
The  presence  of  fifty  layers  on  each  side  of 


74 


REINFORCED    CONCRETE 


the  axis,  instead  of  three,  could  be  assumed, 
and  thus  a  great  amount  of  time  could  be 
tediously  spent  in  calculating  the  "  moment." 
A  formula  has,  however,  been  devised  for  the 
moment  of  inertia  of  all  the  simple  sections, 
and  the  formula  will  be  given  for  each  case. 


about  the  axis  x — x  than  about  Y — y  (Fig. 
99).   The  formula  will  be  I  =  &  ^ 


6  x  123  -  2  x  2-75  x  10-53 


12 
=  333421. 


To   calculate  the  moment  about  the  other 


p—  b- 

—  *•                                    («  —  -D  f 

r 
d 

1 

I 

L 

/         \ 

3      W 

Fig.  9 
I    -   b 

i 

d8.                      !   _ 

12 

b—  >| 

Fig.  92                                             Fi«'  93 
6~d3  -  6'  d'3                      I   =  '7854  r 

12 

1 

t  *~r  n^ 

'igs.  91-97             ^ 

mon  Sections 

^  .,    .                int.-u.TRAi        AXIO  -k 

T7 
t± 

I                 F 
Com 

Ll            \ 

Foments  of             cf 

Fig.  96 
j   _     6  d8  -  26'  d'3 

Inertia                   ^                          J[c' 

Fig.  97 

12 

i.= 


Fig.  94 

•7854  (r4  -  r'4) 


I  =  j  |  b  d3  +  b'  d'3 
-  (6'-  b)d"3\ 


I  -I  {&*-  (b  -i)  (d  -c)3 
+  b'  d'3  -  (b'  -  t)   (d'  -  c')3[ 


axis,  place  the  joist  so  that  it  is  6  in.  deep  and 
12  in.  wide,  as  Fig.  99.     The  inertia  will  be 
flanges  web 

•75  x  63  x  2  +  io-5  x  -53 


-b'— J 
Fig.  95 


o  a3 

Now  the  formula  for  a  rectangle  is  -  ^-, 

12 

fe4 
and  for  a  squara  j^»  an(i  so  t^6  true  value 

of  the  inertia  moment  for  the  section  given 


in  Fig.  90  will  be  I  =  ~~- 

6  x  123 
-12—  -  864' 

It  will  be  seen  that  there  is  some  difference 
in  the  result  obtained  by  the  formula  and 
that  obtained  previously,  this  being  due  to 
the  fact  that  the  number  of  layers  in  the 
first  instance  was  taken  at  three  only.  The 
moments  of  inertia  of  the  common  sections 
are  as  given  by  Figs.  91  to  97. 

In  (ach  case  these  formulae  give  the 
least  moment  of  inertia  in  the  positions 
shown,  and  as  an  example  a  rolled  steel 
joist  will  be  taken  and  the  moment  of 
inertia  calculated  (see  Fig.  98). 

The    moment    of    inertia    will    be    more 


1  n     "                                  —    ^('JLUy. 

L                K  -  A* 

»    _                  1 

^ 

7 

r  ) 

d 

-"72" 

x- 

i 

v 

A 

t  ^> 

/ 

i 

f 

—  ' 

i 

d 

-  lO'J 

Fig.   98. — Calculating  Moment  of  Inertia  of 
Rolled  Steel  Joist 


STRESS    SIMPLY    EXPLAINED 


75 


SECTION    MODULUS    AND    MOMENT    OF 
RESISTANCE 

The  terms  "  section  modulus "  and 
"  moment  of  resistance "  are  often  used 
indiscriminately,  but  although  they  are 


are  equal,  opposite,  and  parallel  forces  form- 
ing a  "  couple." 

Take  one-half  of  the  beam  and  imagine 
the  forces  to  be  acting  upon  it,  as  shown  in 
Fig.  101.  Then  the  rotating  tendency 


Fig.   99. — Calculating  Least  Moment  of  Inertia 
of  Rolled  Steel  Joist 

closely  allied,  they  are  distinct  terms,  and 
have  different  values. 

The  section  modulus  can  be  found  by 
dividing  the  moment  of  inertia  by  half  the 
depth  of  the  section. 

The  moment  of  resistance  can  be  found  by 
multiplying  the  section  modulus  by  the 
modulus  of  rupture  of  the  particular  material 
under  consideration. 

Thus,  the  section  modulus  is  a  fixed 
quantity  for  any  given  section,  whilst  the 
moment  of  resistance  depends  on  the  nature 
of  the  material  employed. 

The  bending  moment  or  tendency  to  cause 
failure  in  a  member  must  be  equalled  or 
exceeded  by  the  moment  of  resistance  of 
that  member ;  and  to  illustrate  this  fact, 
and  also  how  the  section  modulus  is  obtained 
— and  from  this  the  moment  of  resistance — 
will  be  the  next  step. 

Calculating  Section  Modulus.— It  has 


L 


Fig.   100. — Rotating  Tendency  of  Beam,  caused 
by  Reaction  and  Weight 

already  been  shown  that  the  reaction  and 
the  weight  on  a  beam  causes  a  certain 
rotating  tendency,  as  indicated  in  Fig.  100. 
It  has  also  been  shown  that  the  value  of 
these  external  forces  on  the  beam  is  equal 
to  the  amount  of  one  force  multiplied  by 
its  distance  from  the  other  force,  as  they 


Fig.  101. — Forces  Acting  upon  One-half 
of  Beam 

caused  by  the  external  forces  must  be 
equalled  by  the  internal  forces  having  a 
rotating  tendency  in  the  opposite  direction. 
This  rotating  tendency  of  the  internal  forces 
is  offered  by  the  compressional  and  tensional 
resistances  of  the  beam,  which  are  equal  to 
one  another,  opposite  in  direction,  and 
parallel,  as  shown  in  the  diagram  by  c  and  T. 
Then  these  two  resistances  form  a  "  couple," 
and  their  value  to  resist  the  bending  moment 
will  equal  the  amount  of  one  resistance 
multiplied  by  their  distance  apart.  The 
method  of  ascertaining  this  value  will  be 
shown. 

It  will  be  seen  that  the  rotating  tendency 
of  the  reaction  and  weight  is  in  a  "  clock- 
wise "  direction,  whilst  the  tendency  of  the 
internal  forces  is  "  anti-clockwise."  The 
direction  of  the  tension  and  compression 
arrows,  shown  on  the  diagram,  can  be  proved 
in  the  following  manner.  The  firm  arrows 


N  - 

w 
a    4     b 

1 

-f?  — 

-N 

1 

'v\ 

C     I     d 

W1 

'/ 

Fig.   102. — Calculating  Section  Modulus  : 
Beam  Centrally  Loaded 

indicate  the  fibres  on  the  half  of  the  beam 
under  consideration,  and  the  dotted  arrows 
show  those  on  the  other  half  of  the  beam. 
The  top  fibres  are  being  compressed  or  pushed 
together,  and  therefore  the  tendency  of  the 
resistance  is  to  pull  the  fibres  apart  as  shown, 
and  vice  versa  in  the  lower  part  of  the  beam. 


76 


REINFORCED    CONCRETE 


The  amount  of  the  rotating  tendency  of 
the  internal  forces  can  be  reasoned  as 
follows.  Assume  that  a  beam  carries  a 
central  load,  as  Fig.  102,  and  mark  a  piece 
of  the  beam  abed  as  shown.  The  tendency 


Fig.   103. — How  Centrally  Loaded  Beam  tends 
to  bend 

of  the  load  is  to  cause  the  beam  to  bend  in 
the  middle,  and  this  is  shown  in  an  exagger- 
ated manner  in  Fig.  103.  Now  mark  on  the 
beam  the  same  portion  abed  as  shown  on 
the  previous  diagram.  In  order  to  do  this, 
begin  by  marking  the  length  n  n  on  the 
neutral  axis,  as  it  is  known  that  no  change 
has  taken  place  in  the  length  of  the  fibres 
at  this  point,  there  being  no  compression  or 
tension.  Through  these  points  n  n  draw  the 
lines  a  c  and  6  d,  which  must,  of  course, 
radiate  to  the  same  centre  as  that  from 
which  the  curved  beam  was  struck.  It  will 


D 

-N 
12." 


VARYING  INTENSITY 

OF  t)JKLt>$ 

Fig.  104 


parallel  to  the  line  a  c,  as  shown  on  the 
diagram  in  Fig.  103,  the  distance  6  a;  "will 
equal  the  amount  of  compression  that  has 
taken  place  in  the  extreme  fibres  at  the  top 
and  the  distance  x  d  will  indicate  the  amount 
of  tension  that  has  taken  place  in  the  extreme 
bottom  fibres.  The  stress  in  the  extreme 
fibres  must  not  exceed  the  resistance  of  the 
material,  as  otherwise  failure  would  result. 
This  will  demonstrate  the  fact  that  as  the 
extreme  fibres  only  can  be  stressed  to  the 
safe  resistance,  the  full  value  of  the  other 
fibres  can  never  be  realised,  and  the  value 
of  each  layer  of  fibres  decreases  as  it 
approaches  the  neutral  axis. 

This  principle  can  now  be  applied  to  the 
section  of  a  beam  and  the  value  of  the  com- 
pression or  tensional  areas  ascertained.  Take 
the  section  of  a  beam  12  in.  by  6  in.,  as  shown 
in  Fig.  104,  where  the  intensity  of  the  stress 
is  indicated  by  the  graduation  of  the  lines, 
which  become  fainter  as  the  stress  becomes 
less.  Assume  also  that  the  limiting  stress 
on  the  extreme  fibres  is  5  tons  per  square 
inch.  Then  the  stress  at  the  neutral  axis 
is  nil,  and  the  mean  stress  on  the  fibres  is 
2^  tons  per  square  inch.  The  resistance 
must  be  equal  to  the  stress  if  there  is  to  be 
equilibrium,  and  therefore  the  resistance  of 
the  compressional  area  will  be  6  in.  x  6  in. 
x  2£  tons  per  square  inch  =  36  x  2|  = 
90  tons. 


Of  6RAVIJY 


Fig.  105 


Fig.  106 


Figs.   104  to  106. — Calculating  Section  Modulus  of  Beam 


then  be  seen  that  a  c  and  b  d  are  no  longer 
parallel  to  one  another  as  in  the  original 
beam,  or,  in  other  Words,  the  distance 
between  a  and  b  has  become  shorter  owing 
to  compression  and  the  distance  between 
c  and  d  has  become  longer  owing  to  tension. 
Now,  if  a  line  a;  a;  be  drawn  through  n2 


Next,  instead  of  assuming  that  the  stress 
on  the  fibres  varies  in  intensity  from  the 
outer  edge  to  the  neutral  axis,  assume  that 
the  intensity  is  the  same,  but  the  area  over 
which  it  acts  varies  from  the  outer  edge  to 
the  neutral  axis  as  shown  by  the  shaded 
triangles  in  Fig.  105,  and  it  is  found  that  the 


STRESS    SIMPLY    EXPLAINED 


77 


same  result  is  given,  with  the  same  sectional 
area  of  beam.  Here  the  shaded  triangle 
which  shows  the  compressional  area  is  equal 
to  one- quarter  the  sectional  area  of  the 

beam,  or,  in  other  words,  -j-.     Afterwards 

multiply  this  by  the  uniform  maximum  in- 
tensity of  5  tons  per  square  inch,  then 

6  x  12 

— j —  x  5  tons  per  square  inch 


72 


360 


=  90  tons 


bd 
as    before.      This    area    -^    is    called    the 

equivalent  inertia  area,  and  it  simplifies 
the  calculations  to  consider  the  beam  as 
offering  its  resistance  in  this  way. 

There  are  two  resistance  areas,  namely,  the 
compressional  and  the  tensional,  and  it  will 
be  seen  that  they  are  equal.  They  repre- 
sent the  internal  forces  which  are  opposite 
in  direction,  and  their  value  to  resist  stress 
is  the  amount  of  one  multiplied  by  its  dis- 
tance from  the  other,  as  they  form  a 
"  couple."  The  distance  apart  of  the  two 
forces  will  be  the  distance  between  their 
centres  of  gravity  as  they  are  represented 
by  triangles,  and  the  centre  of  gravity  of  a 
triangle  is  situated  at  a  distance  of  two- 
thirds  of  its  height  from  the  apex,  as  shown 
in  Fig.  106.  The  centres  of  gravity  are  then 
d 


each  f  ~  from  the  neutral  axis,  and  their      Tnen  M  R  = 


The  section  modulus  is  usually  expressed 
in  formulae  by  the  letter  Z. 
Calculating  Moment  of  Resistance.— 

Having  ascertained  the  "  section  modulus  " 
and  followed  the  principle  of  the  action  of 
the  internal  forces,  the  actual  resistance  of 
these  forces,  which  not  only  depends  on  the 
area  and  leverage,  but  also  upon  the  strength 
of  the  material,  has  to  be  considered. 

Up  to  the  present,  no  account  has  been 
taken  of  the  strength  of  the  material,  and 
therefore  the  section  modulus  for  any  shape 
section  will  be  constant,  no  matter  what 
material  is  employed,  and  this  is  a  great 
consideration. 
b  d2 

Thus   -JT~  is  tne  section  modulus  for  a 

rectangular  section,  and  the  moment  of 
resistance  can  be  found  from  this,  according 
to  the  strength  of  the  material  employed, 
by  multiplying  the  section  modulus  by  the 
ultimate  or  safe  strength  of  the  material 
per  square  inch,  according  to  whether  the 
actual  or  safe  resistance  is  wanted. 

As  an  example,  ascertain  the  safe  resist- 
ance of  a  rectangular  steel  beam  8  in.  x  2  in. 

b  d2 
Then  M  R    =     „-    x    7£  tons,  or  M  R    = 

j     7        79 

— ^ — ,  where  /  equals  the  limiting  stress  on 

the  fibres,  which  is  7|  tons  per  square  inch. 
2  in.  x  8  in.  x  8  in.  x  7|  tons 


distance    from    one    another    is  §  ~  +f~9 

which  equals  -f  d. 

The  distance  f  d  then  equals  the  lever- 
arm  of  the  internal  forces.     The  area  of  one 

force   =    .    and  the  area  multiplied  by  the 

lever-arm  will  give  the  value  of  the  section. 
This  can  be  expressed  as 

bd  2bd2         bd2 

~~         ~~ 


The  value 


b  d2 


then  gives  the  value  of  a 


rectangular  section,  or,  in  other  words,  its 
"  section  ,  modulus."  It  was  stated  pre- 
viously that  the  moment  of  inertia  divided 
by  half  the  depth  of  the  beam  =  the  section 
modulus.  The  moment  of  inertia  for  a 


rectangular  beam  is  y^-. 


bd2 
=  -7T-  as  before. 


12 


bd*        d 
Then  -J2-  -*-  2 


M  R  =  160  in.-tons.  This,  of  course,  must 
equate  with  the  bending  moment  to  produce 
equilibrium. 

It  has  been  seen  that  the  moment  of  inertia 
divided  by  half  the  depth  of  the  section  gives 
the  section  modulus,  and  the  section  modulus 
multiplied  by  the  limiting  stress  on  the  fibres 
gives  the  moment  of  resistance.  This  is 
expressed  in  the  following  formula,  which  is 

one  greatly  used  in  practice  :    MR   =  / -, 

where  /  =  limiting  stress  on  the  fibres,  I  = 
moment  of  inertia,  and  y  =  half  depth  of 
section.  Upon  working  out  the  example 
just  given  by  this  method,  the  same  result 
will  be  obtained. 

CALCULATING  SAFE  LOAD  ON  BEAM 

The  following  example  should  assist  the 
student  to  understand  the  application  of 
formulae  already  given.  Find  the  central 
load  that  can  be  safely  carried  over  a 
span  of  16  ft,  by  a  rolled-steel  joist  12  in. 


78 


REINFORCED    CONCRETE 


x  6  in.,  with  flanges  and  web  1  in.  thick. 


MB*/=  ,/  =  7|tons,  1  = 

y  1Z  1J 

6  x  12  x  12  x  12  2  x  2|  x  10  x  10  x  10 

~~12~  12~~ 

-  864  -  416-6  =  447-4 

d        12 

v   =  -R   =  -TT   =  6  in. 


then  M  R   = 


7-5  x  447-4 
~6~ 


M  R   =  559-25  in.-tons. 


w  I 


But  B  M  -  M  R,  and  B  M  =  -^  for  central 

load. 

w  I 
Therefore  -r-     =  559-25  in.-  tons 

I  =  16  x  12  =  192  in. 
^^=  559-25 


559-25  x  4 


WEI6HT 


w   =  11-65  tons. 

This  method  is  absolutely  accurate,  and 
is  not  approximate,  as  would  be  the  case 
with  an  empirical  for- 
mula. The  student  is 
strongly  advised  to 
work  out  several  ex- 
amples himself,  which 
will  familiarise  him 
with  the  use  of  the 
formula,  and  impress 
it  upon  his  memory. 
A  good  method  is  to 
select  several  rolled- 
steel  joists  from  a 
manufacturer's  list, 
calculate  the  safe  loads 
for  various  spans,  and 
then  check  the  results 
by  reference  to  the 
table  of  s.afe  loads 
for  the  various  joists 
invariably  given  in 
the  list  of  sections. 


REACTION 


Fig.   107.— Strut 
under  Load 


COLUMNS    AND 
STRUTS 


The  theory  of  long 
columns     and     struts 

unfortunately  is  in  an  unsatisfactory  con- 
dition, as  there  is  a  great  difference  of 
opinion  as  to  the  best  formula  to  use  ;  and 
as  the  formulae  in  general  practice  give 
very  varying  results,  the  student  is  often 
confused  and  doubtful  as  to  the  best  course 


to  adopt;  Most  of  the  formulae  in  use  are 
of  a  more  or  less  complicated  nature,  and 
practically  all  are  empirical  formulae,  con- 
taining some  factor  which  has  been  derived 
from  experiment,  and  which  must  be  taken 


Fig.    108. — Diagram  Indicating  Stress  in  Part 
of  Strut 

for  granted.  Unlike  a  tension  bar,  a  com- 
pression bar  which  is  under  a  certain  stress 
is  apt  to  bend,  and  this  bending  tendency 
has  to  be  resisted,  in  addition  to  the  com- 
pression proper,  as  shown  in  Fig.  107. 

Theoretically,  with  the  stress  passing  abso- 
lutely through  the  centre  line  of  the  strut 
there  is  no  tendency  to  bend,  but  practically 
this  condition  is  never  fulfilled,  and  the 
slightest  deviation  from  the  exact  centre 
will  cause  a  bending  stress.  The  diagram 
in  Fig.  108  shows  a  piece  of  the  strut  at 
A  B  enlarged,  and  the  arrows  indicate  the 
stress.  The  horizontal  line  represents  the 
compression  proper,  with  an  amount  equal 
to  x,  while  the  sloping  line  indicates  the  stress 
due  to  the  bending  tendency,  and  this,  of 
course,  will  vary  in  intensity  at  different 
points  in  the  section  ;  the  edge  of  the  strut 
at  B  will  be  under  great  compression,  while 
the  fibres  at  A  will  be  in  tension.  Now  the 
maximum  intensity  of  compression  will  be 
at  B,  where  the  stress  will  equal  x  x  y, 
while  at  A  the  stress  will  be  at  a  minimum, 
and  will  be  equal  to  a;  —  y,  and  if  y  is  greater 
than  x,  then  the  fibres  at  this  point  will  be 
in  tension.  It  is  clear  that  the  greatest  stress 
x  +  y  must  not  be  greater  than  the  safe 
resistance  to  compression  of  the  material. 
The  principal  difficulty  occurs  in  the  deter- 
mination of  the  bending  tendency. 

Struts  and  columns  are  usually  divided 
into  "  long  "  and  "  short  "  compression  bars, 
and  in  the  short  bars  the  calculations  are 
based  on  the  assumption  that  the  failure  is 


STRESS    SIMPLY    EXPLAINED 


79 


by  direct  crushing  only,  while  long  bars  are 
those  which  fail  by  bending.  A  sudden 
change  has  been  assumed,  although,  of 
course,  this  does  not  actually  happen.  The 
following  is  the  ratio  between  the  least 
diameter  and  the  length  when  the  bars  cease 
to  be  considered  as  short.  Wood,  20 ; 
wrought-iron  and  steel,  10 ;  cast-iron,  5. 
These  ratios  will  be  found  to  vary  greatly 
according  to  different  authorities. 

The  formula  chiefly  used  in  this  country 
for  long  struts  is  that  known  as  "  Gordon's 
Formula,"  which  is  as  follows  : — 

R_  = Ar'    -.  where 


a  =  a  constant  found  by  experiment. 

d  —  the  least  diameter. 

A  =  area  of  the  cross  section. 

re  =  safe  resistance  to  compression  of  the 

material. 

I  =  length  in  same  units  as  d. 

Re  =  total  pressure  in  same  units  as  rc. 


Material 

CVoss 
Section 

Values  of  a 

rc 

*« 
"®  21 
1^ 

ITS 
14 

!•» 

23- 

TIMBER  .  . 

Rectangular    1 
Circular  .  .  j 

4 
250 

i 

250 

i 

100 

13 

cwt. 

WROUGHT- 
IRON 

Rectangular 
Circular  .  . 
Solid 
Hollow    .  . 

4 

1 

1 

2500 

2500 

1000 

LT+  I 

n  J 

4 

900 

1 

900 

1 

360 

-4  tons 

I    U    j 

STEEL 

do. 

do. 

do. 

do. 

1\  tons 

CAST-IRON 

Circular 

1 

1 

1 

Solid 

100 

400 

160 

Circular 

1 

1 

1 

Hollow    .  . 
Rectangular 

200 
3 

800 
3 

320 

3 

640 

:  8  tons 

400 

1600 

X  shaped  .  . 

3 

200 

3 

800 

3 

320 

/ 

In  re  a  factor  of  safety  of  4  only  is  allowed, 
and  therefore,  in  the  case  of  timber  and  cast- 
iron,  it  will  be  advisable  to  use  a  higher 
factor  in  important  work,  and  this  will 
reduce  the  value  of  r^ 


SHEARING    STRESS 

Before  studying  calculations  for  the  value 
of  the  shearing  stress  under  different  con- 
ditions of  loading,  it  will  be  advisable  to 
explain  what  the  expression  "  shearing 


DOTTED 


BEAM 


1 

¥ 

v\ 

'(/' 

i 

1 

Fig.    109.— Beam,    under    Vertical     Shear, 
Assumed  to  Consist  of  Separate  Blocks 


stress "  means.  This  stress  is  actually  a 
sliding  tendency  caused  by  the  opposition 
of  the  load  and  the  reaction,  and  as  the 
beam  or  cantilever  is  the  medium  through 
which  these  two  can  oppose  one  another, 
the  beam  is  subjected  to  this  tendency  both 
vertically  and  horizontally.  As  an  illus- 
tration of  this,  let  it  be  assumed  that  it  is 
possible  to  divide  a  beam  up  into  a  cer- 
tain number  of  parts  by  cutting  vertically 
through  it,  so  that  each  block  is  separate 
from  the  adjoining  block,  and  yet  it  is  pos- 
sible to  put  the  beam  in  position  and  apply 
the  load.  Then  the  load,  in  travelling  along 
the  beam  toward  the  abutments  would  tend 
to  push  down  each  block  and  cause  it  to 
slide  past  the  adjoining  block  as  illustrated 
in  Fig.  109.  This  does  not  actually  take 
place  in  practice,  as  the  beam  would  be 
homogeneous ;  but  the  tendency  is  pre- 
sent, and  must  be  provided  for.  This  is 
known  as  vertical  shear.  There  is  also 


DOJTtD  LlNEt)  6HtW 


Fig.   110. — Beam,   under  Horizontal  Shear, 
Assumed  to  consist  of  Separate  Planks 


a  tendency  to  shear  in  a  horizontal  manner, 
and  this  is  illustrated  in  Fig.  110,  where  the 
beam  is  divided  into  a  number  of  horizontal 
parts  or  planks ;  upon  the  load  being 
applied,  there  is  a  tendency  for  the  beam 
to  assume  the  form  shown  in  an  exaggerated 


REINFORCED    CONCRETE 


manner.  As  the  ends  of  the  planks  coin- 
cided in  the  first  instance,  and  after  the 
load  has  been  applied  they  are  no  longer 
coincident,  it  is  obvious  that  they  have 
slid  past  one  another,  thus  forming  a  hori- 

W- 10  TOKS 


Fig.   111. — Shearing  Stress  in  Cantilever  which 
carries  Concentrated  Load  at  Outer  End 


zontal  shearing  action.  At  all  points  in 
the  beam  the  vertical  and  horizontal  shear- 
ing stress  are  equal,  and  they  will  be  found 
to  be  at  the  maximum  at  the  support.  The 
distribution  of  the  shearing  stress  over  the 
area  of  the  cross  section  is  explained  in  the 
next  chapter  and  need  not  be  considered 
here.  The  amount  of  shearing  stress  is  not 
due  to  any  question  of  leverage,  but  is  due 
to  the  amount  and  nature  of  the  loading, 
and  not  to  the  span,  as  in  the  case  of  the 
bending  moment.  The  conditions  under 
which  it  is  likely  to  become  a  serious  con- 
sideration are  those  where  a  heavy  load 
has  to  be  carried  over  a  short  span,  as  in 
this  case  the  bending  moment  will  be  com- 
paratively small  the  leverage  being  small. 


Dl6TRi&ujED  LOAD-  (TON  pte 
FT  ^UH 


112.  —  Shearing  Stress  in  Cantilever  which 
carries  Uniformly  Distributed  Load 


The  possibility  of  failure  by  shear  is  fre- 
quently ignored  on  account  of  the  fact 
that  the  amount  of  material  required  to 
resist  the  bending  moment  is  usually  such 
that  ample  provision  for  shear  is  given. 
In  the  case  of  heavy  loads  over  a  short  span 


this  does  not  always  follow,  and  consequently 
the  shearing  stress  must  be  calculated  for 
as  well  as  the  bending  moment.  With 
regard  to  the  distribution  of  the  shearing 
stress  over  the  length  of  the  cantilever  or 
beam,  it  must  be  fully  realised  at  the  out- 
set that  this  stress  is  due  to  the  opposition 
of  the  load  and  reaction,  and  consequently 
the  value  of  the  shear  at  any  point  can  only 
be  equal  to  the  load  that  is  travelling 
through  that  point  to  reach  the  abutment. 
As  a  first  example,  consider  a  cantilever 
that  is  carrying  a  concentrated  load  at  its 
outer  end.  Then  at  the  support  the  shear- 
ing stress  will  be  equal  to  the  reaction,  as  at 
this  point  the  total  load  will  be  opposed  by 
the  total  reaction,  and  the  opposition  is 
therefore  at  the  maximum.  If  any  inter- 
mediate point  in  the  cantilever  is  considered, 
it  will  be  seen  that  the  shear  is  still  equal  to 


5TON6  5  TONS 

I    DISTRIBUTED  LOAD  I 
if  I  TON  pEEf  FT  RUN.  I 


^__?TONi 

JTONi 

I 

5TOITS 

-Jr_     -i 


i 


Fig.  113. — Shearing  Stress  in  Cantilever  which 
carries  Uniformly  Distributed  Load  and 
Two  Concentrated  Loads 

the  total  load,  as  in  passing  outward  from 
the  support  no  load  is  passed  over  or  left 
behind,  as  it  were,  until  the  extreme  outer 
end  is  reached.  The  shearing  stress  at  all 
points  will  therefore  be  as  indicated  in 
Fig.  Ill,  where  the  length  of  the  vertical  line 
at  any  point  equals  the  shear  at  that  point. 
If  W  T=  10  tons,  then  the  shearing  stress  at 
the  support  or  any  intermediate  point  equals 
10  tons. 

In  the  case  of  a  cantilever  carrying  a 
uniformly  distributed  load,  the  shearing 
stress  at  all  points  is  somewhat  different,  as 
will  be  seen  on  reference  to  Fig.  112.  In 
this  case  the  shear  will  uniformly  diminish 
from  the  support  to  the  outer  end,  where  it 
will  be  nil.  In  passing  out  from  the  sup- 
port, it  will  be  obvious  that  the  load  will 
be  gradually  passed  over,  as,  for  example, 


STRESS    SIMPLY    EXPLAINED 


81 


at  a  point  1  ft.  from  the  support  1  ton  will 
be  left  behind  and  the  opposition  of  10  tons 
reduced  by  this  amount,  giving  a  shearing 
stress  equal  to  10  -  1  =  9  tons.  When 
5  ft.  has  been  passed  over,  then  5  tons  will 
be  left  between  this  point  and  the  support, 
and  the  shear  will  equal  10  tons  -  5  tons  = 
5  tons,  as  shown  in  the  diagram. 

It  has  been  stated  that  the  shear  at  any 
point  can  only  equal  the  load  that  is  passing 
through  that  point  on  its  way  to  the  sup- 
port, or,  in  other  words,  in  a  cantilever  the 
shear  at  any  'point  is  equal  to  the  load  between 
that  point  and  the  other  end.  Apply  this  rule 
to  a  cantilever  carrying  both  distributed 
and  concentrated  loads  ;  it  will  be  shown 
how  the  shear  is  obtained.  The  example  in 
Fig.  113  shows  a  cantilever  carrying  a 
uniformly  distributed  load  of  1  ton  per  foot 
run  over  a  length  of  10  ft.,  together  with 


»  5 


Fig.    114.  —  Shearing  Stress  in    Beam  which 
carries  a  Central  Concentrated  Load 


two  concentrated  loads  of  3  and  5  tons  res- 
pectively. Beginning  at  the  outer  end  and 
passing  inward,  and  applying  the  rule  that 
the  shear  at  any  point  equals  the  load  be- 
tween that  point  and  the  outer  end,  we  shall 
have  placed  a  distributed  load  of  2  tons 
between  us  and  the  outer  end  by  the  time  we 
have  passed  over  2  ft.,  and  consequently 
there  will  be  a  gradually  increasing  shear 
from  nil  to  2  tons  ;  and  upon  passing  over 
the  2  ft.  mark  the  centre  of  gravity  of  the 
concentrated  load  of  3  tons  will  be  passed 
and  placed  between  us  and  the  outer  end, 
and  the  shear  is  therefore  suddenly  increased 
by  3  tons,  giving  a  total  of  5  tons.  The 
next  5  feet  give  a  gradually  increasing  shear 
of  5  tons,  which  must  be  set  downwards  on 
the  diagram,  and  immediately  on  passing 
the  line  of  the  second  concentrated  load 
another  5  tons  is  added,  giving  a  shear  of 
15  tons.  From  this  point  to  the  support 
we  pass  over  a  distance  of  3  ft.,  and  gradu- 
6 


ally  leave  behind  the  remaining  portion  of 
the  distributed  load,  equal  to  3  tons,  thus 
giving  a  total  shear  at  the  support  of  18 
tons,  which  is  equal  to  the  reaction  at  the 
support.  This  is  clearly  illustrated  on  the 
diagram  referred  to.  The  same  principles 
and  rule  apply  to  any  kind  of  loading, 
whether  partly  distributed  or  partly  con- 
centrated, bearing  in  mind  that  a  distri- 
buted load  causes  a  varying  shear,  repre- 
sented by  a  sloping  line,  and  a  concentrated 
load  causes  a  sudden  increase  in  the  shear 
equal  to  the  amount  of  the  load. 

Beams  require  a  little  more  consideration 
on  account  of  there  being  two  supports, 
and  consequently  a  portion  of  the  loading 
travels  one  way  and  is  opposed  by  one  re- 
action and  the  remainder  travels  in  the 
opposite  direction  and  is  opposed  by  the 
other  reaction.  We  have,  therefore,  a  shear 
towards  the  left-hand  abutment,  caused  by 
the  opposition  of  R1,  and  a  shear  toward  the 
right-hand  abutment  caused  by  R2,  and  the 
shear  at  either  support  will  equal  the  re- 
action at  that  support.  In  the  case  of  a 
beam  carrying  a  central  concentrated  load 
only,  as  illustrated  in  Fig.  114,  the  reac- 

W 

tions  will  each  be  equal  to  -^,  which  in  this 

case  is  -~-  =  5  tons.     To  distinguish  between 

a 

the  shear  towards  the  left-hand  and  right- 
hand  abutments,  a  horizontal  line  is  drawn, 
known  as  the  separation  line,  and  the  value 
of  R1  is  set  up  to  scale  over  this  line  and 
the  value  of  R2  below  as  shown.  The  full 
opposition  will  exist  at  all  points  between 
the  support  and  the  centre  of  gravity  of 
the  load,  as  the  whole  of  the  load  which 
travels  to  the  support  will  be  passing  through 
every  point  in  the  beam,  and  there  is  con- 
sequently no  diminution  in  the  shear  at 
any  point. 

The  rule  for  the  shear  at  any  point  in  a 
beam  can  be  expressed  as  follows :  the 
shearing  stress  at  any  point  in  a  beam  is 
equal  to  the  reaction  at  the  support  minus  any 
load  situated  between  that  point  and  the  sup- 
port selected.  If  the  load  between  the  point 
and  support  selected  is  greater  than  the 
reaction,  then  it  indicates  a  shear  toward 
the  other  support.  As  an  illustration  of  this, 
supposing  the  shear  at  a  point  7  ft.  6  in.  from 
the  left-hand  abutment  shown  in  Fig.  114 
is  to  be  calculated  from  R1.  Then  (R1  - 
the  load  of  10  tons)  =  (5  tons  -  10  tons) 
=  -  5  tons,  thus  indicating  a  shear  toward 


REINFORCED    CONCRETE 


the  right-hand  abutment  which  agrees  with 
the  diagram.  In  the  case  of  a  uniformly 
distributed  load  on  a  beam,  the  shear  at 
either  abutment  will  be  equal  to  the  reac- 
tion at  that  abutment,  and  it  will  gradually 
diminish  to  the  centre,  where  it  will  be  nil, 
as  illustrated  in  Fig.  115.  An  example  of 
combined  concentrated  and  distributed  load- 
ing is  given  in  Fig.  116,  and  if  this  is  ex- 
plained no  difficulty  should  be  encountered 
by  the  student  in  working  out  other  examples, 
however  complicated  the  loading  may  be. 

As  the  reaction  at  the  support  is  equal  to 
the  shear,  it  will  be  necessary  to  calculate 
this  in  the  first  instance.  Therefore  K1  will 

equal  half  the  distributed  load     = 

B 

(3  x  8)  +  (4  x  6)  +  (5  x  3) 
5  tons    +  - 


Loftb  -  |  pn  peg  17- KM 


Fig.    115. — Shearing  Stress   in  Beam  which 
carries  Uniformly  Distributed  Load 

24  +  24  +  15 
5  tons   +   -  -  =  5  tons    +    6^ 


tons    =  11TS7, 


10 
tons. 


Working  out  R2   =  5 


tons    + 


tons    + 


(3  x  2)  +  (4  x  4)  +  (5  x  7) 
10 

=  5  tons   + 


=  5 


6  +  16+35 
10 


10T7^  tons.  E1  +R2  =  total  loading,  therefore 
llfV  +  lOy7^  =  22  tons,  which  is  correct. 
If  we  now  draw  a  horizontal  separation 
line  and  set  up  R1  to  scale  at  the  left-hand 
abutment,  we  shall  have  the  shear  at  this 
point,  and  we  can  proceed  to  find  the  shear 
at  all  points.  After  passing  over  the  first 

2  feet  we  have  reduced  the  shear  by  2  tons, 
giving  9-^  tons  at  this  point,  and  directly 
we  pass  this  mark  we  reduce  the  stress  by 

3  tons,  due  to  the  concentrated  load  leaving 
a  shear  of  6^  tons.     We  now  gradually 


leave  behind  a  further  2  tons  by  the  time 
the  next  concentrated  load  is  reached, 
making  a  reduction  to  4j%-  tons,  and  upon 
passing  the  concentrated  load  we  only  have 
T3o  ton  left  as  shear  toward  the  left-hand 
abutment.  As  the  distributed  load  equals 
1  ton  per  foot  run,  it  will  be  obvious  that 
when  we  have  passed  over  a  further  distance 
of  y'k  ft.,  we  shall  have  no  shear  to  this 
abutment,  and  it  is  therefore  at  this  point 
that  the  separation  line  will  be  crossed 
and  the  shearing  stress  will  be  below  the 
line,  and  thus  towards  the  right-hand  abut- 
ment. This  point  of  crossing  the  line  will 


III  ION 


3  JONS 


2  TO 


LT 


4_ 


5UJED  Lc 
PEE- FT  " 


i* 


Fig.  116. — Shearing  Stress  in  Beam  which 
carries  Uniformly  Distributed  Load 
and  Three  Concentrated  Loads 

be  4^  ft.  from  the  left-hand  abutment, 
and  it  is  important  to  be  able  to  calculate 
this  in  the  case  of  reinforced  concrete  beams 
when  special  shear  members  are  provided 
as  shown  in  the  notes  on  this  work.  In 
continuing  along  the  beam  it  will  be  seen 
that  after  passing  the  point  of  crossing  the 
separation  line  a  distance  of  2T7TT  ft.  will 
be  passed  over  before  reaching  the  load  of 
5  tons,  thus  picking  up  2T7Ty  tons,  and  upon 
adding  the  5  tons  the  shear  will  equal  7TV 
tons.  The  remaining  3  ft.  will  add  a  gradu- 
ally increasing  shear  of  3  tons,  making  a 
total  of  lOy7^  tons  upon  reaching  the  sup- 
port, and  this  amount  is  equal  to  the  re- 
action, and  therefore  is  correct.  Fig.  116 
shows  the  shear  at  all  points  in  a  graphic 
manner,  and  should  assist  the  student  in 
following  the  explanation. 


The  Theory  of  Reinforced  Concrete 


Introductory. — The  theory  of  reinforced 
concrete  is,  unfortunately,  in  a  very  unsatis- 
factory state  at  the  present  time,  and,  in 
fact,  it  may  be  said  that  no  kind  of  universal 
agreement  has  been  arrived  at.  Students 
have  no  definite  course  of  study  that  can 
be  followed  with  the  surety  that  diligent 
application  will  enable  them  to  design  and 
calculate  the  material  in  a  manner  consistent 
with  a  universally  recognised  standard.  The 
desirability  of  standardising  the  methods  of 
design  and  calculation  is  fully  recognised  by 
the  various  Institutes  who  are  using  every 
endeavour  to  bring  about  this  reform  ;  and, 
in  the  meanwhile,  it  is  incumbent  upon  all 
writers  on  the  subject  to  help  matters  forward 
by  adopting  the  recommendations  of  the  lead- 
ing Institutes  as  regards  notation  and  general 
methods. 

For  the  purpose  of  this  treatise,  it  has  been 
decided  to  adopt  the  notation  recommended 
by  the  Concrete  Institute,  this  being  regarded 
by  the  writer  as  the  most  satisfactory  that 
has  been  compiled.  The  working  stresses 
employed  are  those  contained  in  the  second 
xeport  of  the  Royal  Institute  of  British 
Architects. 

A  knowledge  of  stresses  and  strains  is 
necessary  to  every  student  before  an  attempt 
is  made  to  take  up  the  calculations  for  re- 
inforced concrete.  Therefore,  although  these 
notes  apply  to  simple  theory  only,  it  is 
necessary  to  assume  that  the  reader  has 
mastered  the  preceding  chapter  and  possesses 
the  knowledge  referred  to  ;  and  only  those 
explanations  will  be  given  which  are  essen- 
tial to  a  proper  understanding  of  the  methods 
of  deduction.  There  is  a  great  tendency  to 
Tender  formulae  and  explanations  very  com- 
plicated, with  the  result  that  many  students 
possessing  only  a  limited  knowledge  of 
mathematics  are  unable  to  follow  them. 
Bearing  this  in  mind,  the  writer  has  endea- 
voured to  delete  all  unnecessary  complications 
a.nd  to  put  the  matter  in  a  simple  manner, 
which  is  capable  of  being  understood  by  all 
readers.  The  many  different  symbols  used 
in  the  formulae  are  apt  to  look  formidable 
at  first  sight,  but  the  reader  is  advised  to 


make  every  effort  to  become  familiar  with 
them  and  to  endeavour  to  realise  the  full 
meaning  of  each  one. 

It  is  not  sufficient  to  learn  several  formulae 
and  to  be  able  to  put  them  down  from 
memory,  as  this  will  not  enable  them  to  be 
used  intelligently ;  each  one  must  be  under- 
stood and  its  construction  realised.  Indeed, 
the  user  should  be  able  to  construct  his  own 
formulae  when  he  has  fully  grasped  the 
principles  of  force  and  resistance. 

The  calculations  can  be  divided  into  two 
sections,  namely,  (1)  the  determination  of 
the  value  of  the  external  force  or  forces, 
and  (2)  the  determination  of  the  necessary 
resistance  values  to  overcome  these  forces. 

The  value  of  the  force  to  cause  stress  will 
depend  upon  the  nature,  amount,  and  dis- 
position to  the  resisting  member,  and  the 
value  of  the  resistance  will  depend  upon  the 
shape  and  size  of  the  member  and  the 
material  of  which  it  is  composed. 

DATA  FOR   CALCULATIONS 

Ascertaining  Loads  to  be  Carried. — 

There  must  be  some  basis  upon  which  to 
found  the  calculations,  and  the  following 
methods  of  ascertaining  the  loads  to  be 
carried  must  be  taken  into  consideration : 

(1)  The  weight  of  the  structure,  including 
flooring,  plaster,  and  any  applied  decoration 
or  similar  loading. 

(2)  The  superimposed  or  accidental  load, 
which  will  vary  in  amount  according  to  the 
class  of  building  being  designed. 

(3)  Vibration  and  shock  wherever  this  is 
likely  to  occur.     This  will  vary  with  the  class 
of  building,  as  obviously  a  greater  allowance 
must  be  made  in  the  case  of  a  building 
containing  machinery  than  in  the  case  of 
an  ordinary  dwelling  house. 

In  the  case  of  item  No.  1  the  weight  of  the 
concrete  and  steel  may  be  taken  at  150  Ib. 
per  foot  cube.  To  this  must  be  added  the 
flooring  material  or  any  other  weight,  accord- 
ing to  circumstances. 

The  load  per  square  foot  to  be  allowed, 
as  in  item  2,  may  be  taken  on  the  following 
basis  : — 


84 


REINFORCED    CONCRETE 


lb.  'per  sq.  ft. 

Ordinary  dwelling  house,  flats, 
hotel  bedrooms,  and  hospital 
wards  .....  75 

Offices,  schoolrooms,  etc.       .         .  100 

Theatres,  libraries,  concert-rooms, 

banks,  and  shops  .  .  .  120 

Ballrooms  and  drill  halls      .          .  150 

Warehouses  and  stores  vary  ac- 
cording to  use  .  .  .  230-670 

Factories  and  machine  shops.  These 
will  vary  greatly,  according  to 
the  number  and  weight  of 
machines,  and  each  case  must 
have  special  consideration. 

Koofs  generally   ....  50 

The  allowance  for  shock  mentioned  in 
item  3  will  be  necessary  in  the  case  of  build- 
ings which  are  subjected  to  greatly  varying 
loads,  as,  for  instance,  public  buildings, 
factories,  or  workshops,  and  such  allowance 
in  these  cases  should  be  taken  at  one  half  the 
superimposed  load  as  given  above  for  the 
various  buildings.  In  structures  where  the 
vibration  is  caused  by  machinery  or  heavy 
traffic,  as  in  the  case  of  vault  roofs,  the 
allowance  should  be  equal  to  the  actual 
superimposed  load. 

In  the  case  of  columns  and  piers  which 
carry  several  stories  above,  a  certain  reduc- 
tion of  the  load  picked  up  at  the  different 
levels  is  permissible,  as  follows  : — For  the 
part  of  the  roof  or  top  floor  supported,  the 
actual  superimposed  load  calculated  is  taken. 
For  the  next  floor  below  a  reduction  of  10 
per  cent,  is  made,  and  for  the  floor  below 
this  a  reduction  of  20  per  cent,  is  made,  and 
so  on  until  the  floor  is  reached  at  which 
the  reduction  is  equal  to  50  per  cent. 
Below  this  level  all  the  floors  are  taken 
at  50  per  cent,  below  the  superimposed 
loads  in  determining  the  reactions  on  the 
columns. 

This  reduction  must,  of  course,  be  used 
with  discretion,  as  it  is  based  on  the  assump- 
tion that  all  the  floors  would  not  be  loaded 
to  their  full  capacity  at  the  same  time,  and 
therefore,  in  the  case  of  warehouses  and 
some  buildings  where  all  the  floors  may  be 
heavily  loaded  together,  it  is  wide  to  make 
no  reduction  whatever. 

Working  Stresses. — The  following  work- 
ing stresses  are  based  on  the  assumption 
that  the  concrete  is  of  such  a  quality  that 
its  crushing  strength  is  at  least  1,800  lb.  per 
sq.  in.  after  twenty-eight  days,  and  that  the 


steel  has  a  tensile  strength  of  not  less  than 
60,000  lb.  per  sq.  in.  :— 

lb.  per  sq.  in. 
Concrete  in  compression  in  beams 

subjected  to   bending      .          .  GOO 

Concrete  in  columns  under  simple 

compression    ....  GOO 

Concrete  in  shear  in  beams  .  60 

Adhesion    or   grip    of   concrete    to 

metal 100 

Steel  in  tension  ....      16,000 
Steel  in  compression — fifteen  times 

the   stress   in   the   surrounding 

concrete          .... 
Steel  in  shear      ....      12,000 

The  resistance  of  concrete  to  tension  is 
neglected,  as  its  value  is  small,  and,  in 
addition,  it  cannot  be  considered  as  a  reli- 
able material  to  take  such  a  stress  ;  there- 
fore, the  steel  is  calculated  to  take  all  the 
tensile  stress.  The  figures  given  above  for 
concrete  are  based  on  tests  made  with  con- 
crete made  in  the  proportions  of  1  cement, 
2  sand,  and  4  hard  stone,  and  should  different 
proportions  be  adopted  the  stress  may  be 
taken  at  one-third  the  crushing  strength 
twenty-eight  days  after  moulding.  This  may 
be  considered  a  low  factor  of  safety  by  some 
designers,  but  it  must  be  borne  in  mind  that 
the  maximum  strength  of  the  material  is 
not  developed  in  twenty-eight  days. 

If  the  steel  has  a  greater  ultimate  strength 
in  tension  than  60,000  lb.  per  sq.  in.,  the 
safe  stress  may  be  taken  at  one  half  of  the 
stress  required  to  reach  the  elastic  limit  of 
the  material ;  but  in  no  case  is  it  advi?able 
to  calculate  for  a  greater  stress  than  20,000 
lb.  per  sq.  in. 

The  stress  is  considered  as  acting  on  the 
concrete  in  a  uniformly  varying  manner, 
while  in  the  case  of  the  steel  it  is  considered 
as  uniform  over  the  cross  section. 

Elasticity. — The  question  of  the  relative 
elasticity  of  the  two  materials  is  highly 
important,  and  although  there  is  some 
difference  of  opinion  on  this  point,  it  is  now 
generally  accepted  that  the  coefficient  of 
elasticity  for  concrete  in  compression  gauged 
as  above  mentioned  is  constant  and  can  be 
taken  at  one-fifteenth  that  of  steel.  The 
modulus  for  concrete  is  expressed  by  E,., 
and  equals  2,000,000  lb.  per  sq.  in.  The 
modulus  for  steel  is  expressed  by  Es,  and 
equals  30,000,000  lb.  per  sq.  in.  ;  therefore 

E 

T/  =  15.      It  is  highly  important  that  the 


THEORY    OF    REINFORCED    CONCRETE 


student  should  realise  this  point,  as  it  means 
that  at  any  given  distance  from  the  neutral 
axis  the  stress  per  square  inch  on  the  steel  will 
be  fifteen  times  as  great  as  that  on  the  con- 
crete. In  other  words,  the  stress  can  only 
be  proportional  to  the  resistance  where 
equilibrium  is  produced,  and  as  the  steel  is 
capable  of  offering  fifteen  times  as  much 
resistance  to  stretching  as  the  concrete  is  to 
compression,  it  will  require  fifteen  times 
more  stress  in  the  steel  to  produce  the  same 
effect  a^  that  taking  place  in  the  concrete. 

Spans.  —  The  span  to  be  considered  in  the 
calculations  will  not,  of  course,  be  the  clear 
span,  but  the  effective  span,  and  this,  in  the 
case  of  beams,  may  be  taken  as  the  distance 
between  the  centres  of  bearings.  For  slabs 
supported  at  the  ends  only,  the  distance  will 
be  the  clear  span  plus  the  thickness  of  the 
slab  ;  while  for  continuous  slabs  it  becomes 
the  distance  from  centre  to  centre  of  the 
supporting  beams. 

Bending  Moments.  —  The  question  of 
bending  moments  has  been  fully  dealt  with 
in  the  preceding  chapter,  but  it  must  be 
borne  in  mind  that  a  slab  can  only  be  con- 
sidered as  continuous  when  it  is  continued 
over  three  or  more  spans,  and  then  the  spans 
must  be  equal  and  the  supports  at  the  same 
level.  With  a  uniformly  distributed  load 
the  maximum  bending  moment  may  be  taken 


as  +  -r~-  at  the  centre  and  —  ^r^   at  the 

intermediate   supports,    where   W    =   total 
weight  and  I  equals  span. 

BEAMS  WITH  SINGLE  REINFORCEMENT 

Some  little  difficulty  is  experienced  by  the 
student  in  determining  the  moment  of  re- 
sistance of  a  reinforced  beam,  as  there  are 
two  materials  to  be  considered,  each  having 
a  different  modulus  of  elasticity  and  re- 
sistance to  stress.  It  is  necessary,  however, 
to  be  able  to  determine  their  moment  of 
resistance  at  any  point  in  order  that  it  may 
be  shown  that  it  equates  with  the  bending 
moment  at  the  same  point. 

The  bending  moment  represents  the  value 
of  the  external  forces  to  destroy  the  beam 
by  tension  or  compression,  the  former  taking 
place  on  the  lower  part  of  the  beam  and 
the  latter  on  the  upper,  where  the  beam  is 
supported  at  the  ends  only.  If  the  beam  is 
fixed  at  the  ends,  then  the  beam  will  be 
called  upon  to  resist  compression  in  the  lower 
part  and  tension  in  the  upper  part  for  a 
certain  distance  from  the  fixed  ends,  but  for 


the  purpose  of  this  present  explanation  it  will 
be  considered  that  the  beam  is  supported 
only. 

There  are  several  formulae  for  the  moment 
of  resistance  of  a  reinforced  concrete  beam, 
but  it  is  not  sufficient  to  give  these  without 
explaining  the  principle  of  their  construction. 

It  is  shown  in  the  preceding  chapter  that 
the  compressional  and  tensional  resistances 
must  be  equal  to  one  another,  and  that  they 
are  acting  in  opposite  directions  and  are 
parallel,  thus  forming  a  "  couple,"  and  the 
value  of  the  moment  of  resistance  of  the 
whole  section  is  the  amount  of  one  resisting 
power  multiplied  by  its  distance  from  the 
other. 

The  resistance  to  compression  is  supplied 
by  the  concrete  above  the  neutral  axis,  while 
the  resistance  to  tension  is  supplied  by  the 
steel  in  the  bottom  of  the  beam,  the  concrete 
below  the  neutral  axis  being  neglected,  as 
shown  in  Figs.  117  and  118.  Here  it  will  be 
seen  that  the  stress  on  the  concrete  is  shown 
as  being  of  a  uniform  intensity  acting  over 
an  equivalent  inertia  area  which,  as  pre- 
viously explained,  is  equal  to  a  varying 
intensity  over  the  whole  of  the  compressional 
area.  Hence  the  equivalent  compressional 

area  will  equal  -^-,  where  6  =  the  breadth 

of  the  beam  in  inches  and  n  =  the  distance 
of  the  neutral  axis  from  the  compressed  edge. 
The  safe  compressional  stress  per  square  inch 
on  the  concrete  is  expressed  by  c,  and  the 
value  of  the  compressional  resistance,  there- 

C  t)  YL 

fore    =       — .     The  safe  resistance  of  the 

A 

steel  in  tension  per  square  inch  is  expressed 
by  the  symbol  t,  and  the  sectional  area  in 
square  inches  by  At,  therefore  the  value  of 
the  tensional  resistance  equals  At  x  t. 
The  total  compression  and  total  tension 

are  equal,  and  therefore  — ~-   =  t  At.     These 

two  equal  resistances  or  internal  forces  act 
in  opposite  directions,  and  the  amount  of 
one  of  them  must  be  multiplied  by  the  dis- 
tance between  them  to  find  the  moment  of 
resistance.  The  centre  of  gravity  of  the 
compressional  area,  at  which  point  the  com- 
pressional resistance  can  be  considered  as 
being  concentrated,  will  be  situated  at  a 

distance  of  «  from  the    top  edge,  or  at  a 
o 

distance  of  f  n  from  the  neutral  axis.  The 
tensional  resistance  will  be  acting  at  a 


86 


REINFORCED    CONCRETE 


point  equal  to  d  —  n  from  the  neutral  axis 

*i 

or  d  —  -x  from  the  centre  of  gravity  of  the 

compressional  area.     Therefore  the  moment 
c  b  n 


of  resistance  = 


('-8 


or  t  At  x 


d  —       ,  and  this  must  be  equal  to  the 

c  b  n 


( 

bending  moment,  B  M.    Thus  B  M  =      ^ 

f          n\  f          n\ 

x  [  d  —  o  I   or  B  M  =  Z  A(   x  I  «  —  «  I  • 

V         «>/  v         «*/ 


Before  any  use  can  be  made  of  these 
formulae  it  will  be  necessary  to  establish  some 
relation  between  certain  factors  which  are 
variable  and  unknown,  namely,  c,  t,  and  n. 

The  factors  6,  d,  and  At  can  be  settled  by 

b 


'i  AX  15 


will  require  fifteen  times  as  much  stress  on 
the  steel  as  that  on  the  concrete  to  produce 
this  result  if  the  two  materials  are  situated 
at  the  same  distance  from  the 'neutral  axis. 
This  is  illustrated  in  Fig.  119,  where  it  is 
assumed  for  the  moment  that  the  neutral 
axis  is  situated  at  the  centre  of  the  depth  ; 
then  the  permissible  stress  on  the  concrete 
is  600  Ib.  per  sq.  in.,  and  the  stress  on  the 
steel  will  equal  c  x  m,  which  equals  600  x  15 
=  9,000  Ib.,  m  being  the  ratio  between 
Es  and  Ec. 

It  must  be  clearly  understood  that 
although  the  deformation  does  not  actually 
take  place  if  the  beam  is  properly  designed, 
there  is  a  tendency  for  it  to  do  so,  and  conse- 
quently the  resistance  has  to  be  exerted  by 
the  materials  to  overcome  the  tendency. 


CEMTRE  OP 
COMPEt55IONAt 


Fig.  117  Fig.  118 

Figs.    117  and  118.— How  a  Single  Reinforced  Beam  Resists  Compression  and  Tension 


the  designer  himself,  but  the  value  of  n,  t, 
and  c  will  be  affected  by  the  relation  or  pro- 
portion of  At  to  6  d,  and  as  there  are  two 
unknown  factors  in  each  formula  it  would  not 
be  possible  to  proceed  beyond  a  certain  point. 
Again,  it  will  not  be  possible  to  stress  both 
the  steel  and  the  concrete  to  the  permissible 
limit  unless  they  are  in  the  correct  proportion 
to  one  another.  The  two  materials  have  a 
different  coefficient  of  elasticity  which  in 
the  concrete  is  expressed  by  Ec.  and  in  the 
steel  by  Es,  and  the  ratio  between  them  = 
E, 


E.. 


=  15,  and  this  is  an  important  point  in 


the  determination  of  the  stresses.  It  signifies 
that  in  the  case  of  deformation  taking  place 
where  the  concrete  is  compressed  by  a  cer- 
tain stated  amount  (say,  1  in.),  while  the 
steel  is  stretched  by  the  same  amount,  it 


The  permissible  stress  on  the  steel  is  16,000 
Ib.,  and  therefore  if  the  proportion  between 
the  two  materials  is  such  that  the  neutral 
axis  is  at  the  centre  of  the  depth,  the  steel 
cannot  be  used  to  its  fullest  capacity.  It 
will  be  necessary  to  have  the  proportion 
such  that  the  neutral  axis  is  higher  up  the 
beam,  and  thus  farther  from  the  steel,  be- 
cause the  stress  varies  directly  as  the  dis- 
tance from  the  neutral  axis,  and  thus,  in 
addition  to  the  ratio  m,  it  is  necessary  to 
consider  the  relative  distances  n  and  d  —  n. 
If  the  steel  is  2  in.  from  the  neutral  axis 
while  the  concrete  is  only  1  in.  from  the 
same  point,  then  the  stress  on  the  steel  will 
be  equal  to  twice  that  in  the  concrete  ;  or, 
in  other  words,  t  =  c  x  2.  The  same  rule 
will  apply  if  it  is  said  that  the  stress  in  the 
steel  multiplied  by  n  will  equal  the  stress 


THEORY    OF    REINFORCED    CONCRETE 


in  the  concrete  multiplied  by  d  —  n,  or 
I  x  n  =  c  x  d  —  n.  Now  taking  into  con- 
sideration the  ratio  m  and  the  distance  from 
the  neutral  axis,  the  result  is  t  n  =  m  c  (d  —n). 
It  is  allowable  to  reduce  this  to  give  a 
relative  proportion  for  n  as  follows  :  — 

in   =  m  c  (d  —  n) 
tn   =  mcd  —  men 
tn   +  men  =  mcd 
n(t   +  me)    =  mcd 

mcd  n  me 


t  +  me 


"" 


t  +  me 


In  the  case  of  this  formula  there  are  cer- 
tain factors  known  if  c  and  t  are  considered 
as  representing  the  permissible  stress  on  the 
concrete  and  steel  ;  thus,  m  =15,  c  =600  Ib., 


AX  15 


Fig.  119.  —  Diagram  Showing  Proportionate 
Stresses  above  and  below  Neutral  Axis  to 
Produce  Deformation 

and  t  =  16,000  Ib.  With  the  aid  of  these  it 
is  possible  to  deduce  n  as  a  definite  propor- 
tion of  d,  as  follows  : — 

mcd 

n  =  — 

t  +  me 

15  x  600  Ib.  x  d 
n  =  rt 


n  = 


16,000  Ib.  +  (15  x  600  Ib.) 

9,000  d 

25,000 


n  =    -~~  d  =  n  —  -36  d. 

Now  this  is  of  great  value,  as  it  shows  that 
in  the  case  of  a  well-designed  beam  where 
t  and  c  are  at  the  permissible  limit,  then  n 
=  -36  d.  It  does  not  necessarily  follow  that 
c  and  t  will  be  equal  to  the  values  given 
above,  as  it  has  previously  been  stated  that 


this  will  only  occur  when  At  and  6  d,  the  areas 
of  the  steel  and  concrete,  bear  a  certain  pro- 
portion to  one  another. 

The  next  step,  then,  is  to  determine  this 
proportion,  and  see  how  the  neutral  axis  will 
vary  if  another  proportion  is  employed. 

The  ratio  of  the  area  of  the  tensile  rein- 
forcement to  the  area  b  d  is  expressed  by  the 

symbol  r,  therefore  r  =  r-v 

Now  it  has  already  been  seen  that  t  At 

cbn  cbn  At 

=  -s— ,  so  that  At   =  -  — .    If  r   =    j-j 

2    '  2t  bd 

cbn  cbn 

then  r  also  equals    0  . ,  , .  as  -^rr  is  the 
Alba  At 

equivalent  of  A<  and  can  be  substituted  for 

cbn 
it.       In    the    expression  r   =     0.77,   the 

symbol  b  can  be  eliminated,  as  it  occurs  in 

c  n 
both  cases,  thus  reducing  it  to  r  =  ^~TJ- 

An  expression  is  now  obtained  which  will 
give  the  ratio  of  the  steel  to  the  concrete  to 
develop  certain  given  values  for  c  and  t. 
In  the  first  place,  consider  the  case  where 
c  =  600  Ib.  and  t  =  16,000  Ib.  as  above, 
where  it  was  seen  that  n  =  -36  d,  then  c,  n, 
t,  and  d  are  known,  thus — 

600  x  -36 


r   = 


T     = 


r  •- 


2  x  16,000  x  1 
216 

32,000 

27 

=  -00675- 


It  will  be  seen  by  this  that  if  the  greatest 
permissible  stress  is  represented  by  c  =  600 
Ib.  and  t  =  16,000  Ib.,  then  n  =  -36  d  and 
r  =  -00675  or  At  =  -00675  b  d. 

Applying  the  Formulae.  —  Befoie  pro- 
ceeding to  the  derivation  of  any  more 
formulae,  it  will  be  as  well  to  apply  those 
obtained  up  to  the  present  and  become 
more  or  less  familiar  with  their  use,  to  pre- 
vent the  subject  from  becoming  too  com- 
plicated for  the  student  who  is  dealing  with 
symbols  and  reasoning  which  are  quite  new 
to  him. 

Briefly  summarised,  the  following  have 
been  deduced  :  (1)  The  total  compression 

cbn 
and  total  tension  are  equal,  expressed  —  ^~ 

=  t  At.  (2)  The  moment  of  resistance 
equals  either  of  these  multiplied  by  d  -  -, 
which  is  the  lever  arm  of  the  internal  forces, 


88 


C  u  71 

expressed  M  R  =     «      x 


REINFORCED    CONCRETE 


-  ^  ) ,  or  M  R 


=  Z  At  x  (  d  -  £  J .  (3)  These  must  be  equal 
to  the  bending  moment,  expressed  B  M  = 
'•-.  £)  or  BM  tAt  x 


At   =  -00675  x  8  x  12-5 

At   =  -00675  x  100 

At    =  -675  sq.  in. 

The  moment  of  resistance  can  now  be  cal- 
culated according  to  formulae  in  deduction 
No.  2,  namely  : — 

cbn       /  _       n> 
MR   =  - 


(  d  -    !  j.     (4)  The  position  of  the  neutral         MR   = 


600  x  8  x  4-5 


axis  is  dependent  on  the  coefficients  of  elas- 
ticity, m  =  15,  and  the  values  of  c  and  t, 

expressed  n   =  7—     — ,  where  c  =  600  lb. 

t    "T"  7/1  C 

and  t  =  16,000  lb.,  then  n  =  -36  d.  (5) 
The  ratio  of  the  steel  reinforcement  to  the 
area  of  the  beam  will  depend  on  the  values 

c  n 
of  c  and  t,  and  will  be  expressed  r  =   2~Td' 

when  c  =  600  lb.  and  t  =  16,000  lb.  ; 
then  r  =  -00675  or  At  =  -00675  b  d. 

Calculating  Amount  of  Reinforce- 
ments.— As  a  first  example,  assume  a  con- 
crete beam  12^  in.  deep  and  8  in.  wide.  It  is 
required  to  find  the  amount  of  steel  neces- 
sary to  reinforce  the  beam  if  the  working 
stresses  are  to  be  c  =  600  lb.  and  t  =  16,000 
lb.  per  sq.  in.  Also  calculate  the  moment 
of  resistance  of  the  beam  when  so  reinforced, 
and  find  what  uniformly  distributed  load  it 
will  carry  over  a  span  of  15  ft.  if  the  ends 
are  supported  only. 

The  values  of  c  and  t  being  known,  the 
first  step  is  to  ascertain  the  position  of  the 
neutral  axis.  By  using  the  formula  in 
deduction  No.  4,  it  will  be  found  n  =  -36  d. 
Therefore  n  =  -36  x  12-5 

n  =  4-5  in. 
Work  this  out  by  means  of  the  formula 

mcd      .  .          . 

n  =  -  for   the    purpose    01    getting 

t  •}-  m  c 

quite  familiar  with  the  use  of  same,  then  n  = 

15  x  600  x  12-5  112,500 

16,000  +  15  x  600    ''  25,000  " 

in.,  as  before. 

Having  derived  the  value  of  n,  the  amount 

C  YL 

of  steel  willjbe  given  by  r  =  ^j,  and  this 

should  work  out  at  -00675,  as  c  =  600  lb. 
and  t  =  16,000  lb. 

600  x  4-5 

=  2  x  16,000  x  12-5 
2,700 
400,000 
r   =  -00675 
At  therefore  equals  -00675  b  d. 


I  4-5\ 

(12-5 -IT) 


~ 


M  R   =  — £-    x  (12-5  -  1-5) 

MR   =  10,800  x  11 
M  R   =  118,800  in.-lb. 
Again,  working  the  moment  of  resistance 
according  to  the  tensile  reinforcement, 

M  R  =  t  At  x  ( d  - 

/  4-J 

MR   =  16,000  x  -675  x  ( 12-5  - -j 

MR   =  16,000  x  -675  x  (12-5  -1-5) 

MR   =  16,000  x  -675  x  11 

MR   =  10,800  x  11 

M  R   =  118,800  in.-lb.,  as  before. 

As  the  moment  of  resistance  equals  118,800 
in.-lb.,  the  bending  moment  must  not 
exceed  this  amount,  and  the  next  step  is  to 
ascertain  what  weight  would  produce  this 
bending  moment. 

In  the  case  of  a  uniformly  distributed  load 

with  supported  ends  B  M   =  -g-,  therefore 

118,800  in.-lb.  =  -g-.     w   is   the   unknown 

quantity,  and   I   =  the  span  in  inches    = 
15  x  12  =  180. 

118,800  in.-lb.  =  — 


118,800  x  8 


w  " 

w   =  5,280  lb. 

w   =  47-14  cwt. 


The  beam  will  therefore  require  -675  sq.  in. 
of  steel  reinforcement,  and  safely  carry  a 
load  of  47-14  cwt.  over  a  span  of  15  ft. 

Calculating  Beam  for  Certain  Load. 
— In  this  example,  the  size  of  the  beam  was 
given  and  the  reinforcement  and  the  load 
were  calculated,  for  the  sake  of  applying 
the  deductions  previously  made.  In  actual 
practice  it  is  usually  required  to  design  a 
beam  to  carry  a  certain  load  or  to  check 
a  given  beam  and  ascertain  if  the  safe 
stresses  on  the  concrete  and  steel  have  been 
exceeded.  As  an  example  of  the  latter 


THEORY    OF    REINFORCED    CONCRETE 


89 


instance,  the  beam  just  dealt  with  can  be 
calculated,  when  the  stresses  should  agree 
with  those  arranged  in  the  question,  namely 
c  =  600  Ib.  and  t  =  16,000  Ib.  The  bend- 
ing moment  to  be  dealt  with  is  118,000 
in.-lb.,  and  as  shown  in  deduction  No.  3, 


BM  =  -o     x 


/  W\ 

(  d  ~~3J- 


The  value  of  c  will 


therefore   be   found   by  the   formula   c    = 
2BM 


2  x  118,800 


x  I  a  —  „ 


c  = 


8  x  4-5  x 
237,600 


396 


=  c  =  600  Ib. 


This  amount  is  correct,  and  the  stress  in  the 
steel  can  next  be  calculated  as  follows  : — • 


BM   —  t  At  xid  —  ^  J ,  therefore 
BM 


t   = 


A, 


t   = 


118,800 


t   = 


118,800 


7.4.25 


_ 

—  t  —  16,000  Ib. 


This,  again,  agrees  with  the  stress  allowed, 
and  thus  the  two  formulae  are  obtained 
which  will  give  the  stresses  in  the  beam 
when  this  has  to  be  checked  after  it  has 
been  designed. 

In  the  actual  designing  of  the  beam,  the 
load  will  be  a  definite  quantity  according 
to  circumstances,  and  the  bending  moment 
will  therefore  require  to  be  calculated  in  the 
first  instance.  It  will  be  necessary,  there- 
fore, to  have  some  equation  which  will  give 
a  suitable  size  beam  to  resist  such  bending 
moment,  and  also  some  method  of  obtain- 
ing the  position  of  the  neutral  axis  when 
the  values  of  c  and  t  are  unknown,  such  as 
would  be  the  case  in  designing  to  resist  a 
given  bending  moment. 

The  next  step  is  to  establish  some  relation 
between  6,  d,  and  the  bending  moment. 
The  draft  regulations  of  the  London  County 
Council  limit  the  minimum  depth  of  a  beam 
to  one-twenty-fourth  of  the  span,  but  as  the 
compressional  resistance  depends  on  the  area 
of  concrete  above  the  neutral  axis  and  the 
resistance  must  be  equal  to  the  bending 


moment,  it  is  preferable  to  calculate  from 
the  bending  moment  direct  without  regard 
to  the  span.  Referring  to  deduction  No.  3, 

begin  again  with  the  equation  B  M  =  —  ~  —  x 


(  d  -  |Y 


Thus  2  B  M  =c  x  b  x  n  x 


(d  -  | 


The  value  of  c  can  be  taken  at  600  Ib.,  and  n 
should  equal  -36  d  if  the  economical  per- 
centage of  reinforcement  is  to  be  employed, 
this  leaving  only  two  unknown  factors  b  and 
d.  If  b  is  settled  as  some  definite  proportion 
of  d,  then  this  proportion  can  be  substituted 
and  d  expressed  as  a  fraction  of  B  M.  A 
well-proportioned  beam  is  frequently  that  in 
which  b  =  -6  d,  and  this  will  be  taken  as  the 
value,  hence  — 


2BM  =  c  x  &  x  n  x  [d  —  7; 


2  B  M  =  603  x  -6  d  x  -36  d  x  (d- 


2  B  M  =  600  x  -6  d  x  -36  d  x  (d— 12  d) 
2  B  M  =  600  x  -6  d  x  -36  d  x  -88  d 
2  B  M  =  600  x  -19008  d3 
2  B  M  =  114-048  d3 
B  M  =  57-024  d3 
BM 


57-024 


d 


3/ 

3  V 


BM 

57-024 


To  simplify  matters  and  save  a  great 
number  of  figures  in  the  calculations,  it  will 

be  sufficient  to  use  d  =  ?/ ,  the  dele- 

V      57 

tion  of  the  decimal  figures  in  this  case  making 
so  little  difference  that  it  is  inappreciable. 
The  writer  is  aware  that  there  are  many 
students  who  have  forgotten  the  method  of 
finding  the  cube  root  of  a  number,  but  when 
this  is  the  case  the  student  should  use  a 
table  and  thus  overcome  any  difficulty  on 
this  point  until  such  time  as  he  shall  again 
become  conversant  with  the  method.  In 
the  absence  of  a  table,  the  cube  root  can  be 
found  by  trial,  but,  needless  to  mention, 
this  is  somewhat  of  a  tedious  method. 

To  illustrate  the  application  of  the  formula, 
it  will  be  advisable  to  work  out  an  example, 
as  before,  the  load  and  span  being  given. 
It  is  required  to  design  a  beam  supported  at 
the  ends  to  carry  a  load  of  650  Ib.  per  foot 
run  over  an  effective  span  of  15  ft.  It  is 
assumed  that  this  load  includes  the  weight 
of  the  beam  itself,  and  the  safe  stresses  are 
to  be  600  Ib.  and  16,000  Ib. 


9o 


REINFORCED    CONCRETE 


The  total  load   =  650  Ib.   x   15  =  9,750  Ib. 

id       9750  x  180 
Ihe  bending  moment  =  -Q-  =  -     — ~ — 

B  M  =  219,375  in.-lb. 

3  A 
Therefore  d  =  //- 


'219375 


_57 

d  =    ^3848 

d  =  15-7  in. 
The  breadth  should  be  -6  d,  therefore 

b  =  -6  x  15-7  in. 

b  =  9-4  in. 

The  size  of  the  concrete  beam  itself  has 
therefore  been  found,  and  the  amount  of 
reinforcement  is  now  required. 

According  to  deduction  No.  5,  the  ratio  of 
steel  to  concrete  should  be  -00675.  Therefore 
At  =  -00675  b  d,  b  d  =  147-58  sq.  in.,  and 
At  =  -00675  x  147-58.  At  =  -996  sq.  in. 
— say  three  y|-in.  diameter  round  bars. 
This  will  give  the  nearest  area,  always  bear- 
ing in  mind  that  there  should  be  an  excess 
rather  than  a  deficiency.  The  area  of  one 
y£-in.  diameter  bar  =  -3712  sq.  in.,  and  the 
total  area  of  the  three  bars  =  1-1136  sq.  in. 
The  beam  as  designed  has  an  effective 
depth  of  15-7  in.  To  this  must  be  added, 
say,  2  in.  of  concrete  to  cover  the  bars, 
giving  a  total  depth  of  17-7  in.  The  com- 
pleted section  should  therefore  be  :  depth, 
17f  in. ;  breadth,  9|  in.  ;  reinforcement, 
No.  3  y£-in.  diameter  round  rods. 

Checking  the  Beam.— In  the  case  of 
beams  which  have  to  be  checked  for  the 
purpose  of  ascertaining  the  values  of  c  and 
t,  the  necessary  formulae  up  to  a  certain 
point  are  at  hand,  but  some  further  ex- 
planations and  deductions  are  required  to 
make  the  data  complete.  As  previously 
shown,  the  stress  in  the  concrete  can  be 

2BM 

found  by  the  formula  c  =  -  — r> 

»-*M) 

and  the  stress  in  the  steel  can  be  ascertained 
by  t  =  -  „,-     The  difficulty  in 


the  case  of  these  formulae  will  be  that  of 
finding  the  value  of  n.  Previously,  the 
position  of  n  has  been  said  to  be  dependent 
on  the  coefficients  of  elasticity  and  the 
values  of  c  and  t. 

In  the  case  of  checking  a  beam  already 
designed  c  and  t  are  unknown,  and  the 
formula  cannot  be  used,  and  if  the  ratio  of 
steel  to  concrete  is  not  -00675,  then  n  will 


not  equal  -36  d.  The  position  of  the  neutral 
axis,  being  dependent  upon  the  comparative 
values  of  c  and  t,  must  also  be  dependent 
upon  the  proportion  of  steel  to  concrete, 
and  as  both  of  these  will  be  given  in  the 
beam  as  designed,  a  value  for  n  must  be 
deduced  accordingly. 

It  has  already  been  shown  that  t  n    = 

,  . .       ,       t       m  x  (d  —  n) 
cm  (a  —  n),  and  therefore  -  =  —  — -. 


cbn 
Again,    —  2 

9  t  A 


=   t  At,    and    therefore   n    = 


Now  t  and  c  are  unknown,  in  which 

C  0 

case    substitute   the    value    for  -,    namely, 


m  (d  —  n) 


Then  n   = 


2  At  m  (d  —  n) 


and 


b  n 

bn2  =  2  At  m  (d  —  n).  Here  n  is  on  both 
sides  of  the  equation,  and  it  must  be  reduced 
as  follows  :  — 

b  n2   =  2  At  m  (d  —  n) 
bn2   =  2  Atmd  —  2  Atmn 
b  n2   +  2Atmn=2Atmd 
Divide  both  sides  by  6,  then 

2  Atmn       2  Atmd 
^    + 1—  — T- 


Add 
then 


(A,  wiV 
^-) 


to  both  sides  of  the  equation, 


2  At  m  n 

~b ' 


At  m\2    2  Atm  d 


n  + 
Therefore 


A,  my  _ 

b   I 

At  m 


(Atm 
\   *> 


+1- 


+ 


/2 

•v 


~Atmd         At  m 


2Atmd 


+ 


(A1  m\a 


m 


Finding  Position  of  Neutral  Axis.— 

A  formula  is  now  to  hand  for  finding  the 
position  of  the  neutral  axis  in  the  case  of  a 
beam  which  has  been  designed,  where  the 
stresses  may  or  may  not  be  in  accordance 
with  the  permissible  limits. 

Assume,  as  a  first  example,  that  the  beam 
given  in  Fig.  120  has  been  designed  to  resist 
a  bending  moment  of  219,375  in.-lb.,  and  it 
is  required  to  ascertain  if  the  stress  in  the 
concrete  and  steel  is  in  accordance  with  the 
values  allowed,  namely  600  Ib.  and  16,000  Ib. 
This  is  actually  the  same  beam  that  was 
calculated  in  the  previous  example,  and  it 
should  be  noted  that  the  area  of  the  steel 


THEORY    OF    REINFORCED    CONCRETE 


reinforcement  required  was  only  -996  sq.  in.  ; 
but  it  was  increased  to  1-1136  to  make  it 
possible  to  use  rods  of  a  practical  size,  and 
it  is  for  the  latter  area  that  it  must  now  be 
calculated.  The  sizes  of  the  concrete  are 
also  the  practical  sizes,  such  as  would  appear 
on  a  drawing  of  the  beam,  when  the  theo- 
retical sizes  would  not  be  available  to  anyone 
checking  the  stresses  in  the  ordinary  way. 
The  first  step  is  to  ascertain  the  position 


n 


R 


Fig.  120. — Finding  Position  of  Neutral  Axis 

of  the  neutral  axis,  and  this  will  be  done  by 
means  of  the  formula  given  above,  namely  : — 


»  =  V 

••V- 


2  At  md 


V    b   ) 


2  x  1-1136  x  15  x  15-75 
9-5 

1-1136  x  15 
9-5 


•/l-1136xl5V 
V        9-5       ) 


n  = 


526-176 


16-704\2       16-704 


9-5        '_  \ .9-5   /  9-5 

n  =     N/55-386  +  (1-758)2  -  1-758 
»=     v/58-476|-l-758 
n  =  7-64  -  1-758 
n  =  5-882  in. 

It  will  be  seen  that  this  result  differs 
slightly  from  that  which  would  have  been 
obtained  by  taking  n  =  -36  d,  as  in  the 
latter  case  this  equals  -36  x  15-75  =  5-67  in., 
and  this  difference  has  arisen  on  account  of 
At  and  b  d  having  been  altered  to  give  prac- 
tical sizes.  Having  now  obtained  the  value 


of  n,  the  values  of  c  and  t  can  be  found  as 
follows  : — 

2BM 

c   =  - 

b  n  : 


c    = 


2  x  219375 


c   = 


9-5  x  5-882  x 

438750 

9-5  x  5-882  x 
438750 


13-70 


A<  x  ( d  -  o 


~  770-57 
c   =  569  Ib. 

This  is  below  the  permissible  stress  of  600  Ib. 
per  sq.  in.,  and  the  value  of  t  may  now  be 
determined  : — 

BM 

t   =  - 


t  = 

t  = 

t  = 
t 


219375 


1-1136 

219375 


"•(' 


5-882 


1-1136  x 
219375 
15-356 
14285  Ib. 


13-79 


This  value,  again,  is  well  below  the  per- 
missible stress,  and  consequently  the  beam  is 
quite  satisfactory  for  the  load  it  has  to  carry. 

It  is  interesting  to  note  that  the  area  of 
steel  required  theoretically  was  increased  by 
about  12  per  cent,  to  give  a  practical  area, 
and  this -reduced  the  stress  on  the  steel  per 
square  inch  by  about  10  per  cent.,  and  the 
stress  on  the  concrete  by  about  5  per  cent. 

In  the  draft  regulations  of  the  London 
County  Council,  it  will  be  seen  that  the 
formula  for  finding  the  position  of  the 
neutral  axis  is  given  as  : 

n  =  [  v''m2  r2  +  2  m  r)  —mr]d 
and  a  note  in  explanation  of  this  should  be 
of  interest  to  those  who  are  not  sufficiently 
advanced  to  trace  the  construction  and 
evolution  of  the  formula.  The  symbol  r,  as 
previously  stated,  equals  the  ratio  of  steel  to 
concrete,  and  the  formula  is  constructed  to 
use  this  ratio  for  finding  the  position  of  n 
when  c  and  t  are  unknown.  Upon  reference 
to  the  reasoning  employed  in  deducing  the 
previous  formula  for  the  position  of  n,  it  will 
be  seen  that  b  nz  =  2Atm(d  —  n).  Therefore 

2  At m  (d  —n) 

n2    =  -      — r- 


REINFORCED    CONCRETE 


Now  in  this  case  At  is  also  unknown,  and  is 
simply  available  as  a  proportion  of  the  con- 
crete, such  proportion  being  expressed  as  r. 
We  can  therefore  substitute  this  equivalent 
and  obtain 

2     _2rbdm(d  —  n) 

Hence — 

n2   =  2rd2m—2rdmn 
n2  +  2r  dmn  =  2r  d2  m 

To  both  sides  of  the  equation  add  (r  d  m)2. 

Then 

n2    +  2rdmn    +  (r  d  m)2    =  (r  d  m)2    + 

2rd2m. 

The  left-hand  side  of  the  equation  can 

now  be  simplified  as   before,  because  n   + 

r  d  m  is  the  square  root  of  same,  and  therefore 

(n  +  r  dm)2  can  be  substituted  as  its  value, 

giving  (n  +rdm)2   =  (rdm)2   +  2rd2m. 

Hence — 

n+rdm=*J(rd  m)2  +2rd2m 
n  =  \/  (r  d  m)2  +  2rd2m  —rdm 
n  =[v/(r  m)2  +  2  r  m  —  r  m]  d 
This,  of  course,  is  the  same  as  the  formula 
given    by    the    London    County    Council, 
namely : 

n  =  [v/wi2  rz  +2mr  —  mr\d 
In  the  previous  notes  it  was  shown  that 
when  r  =  -00675,  which  is  the  economical 
proportion  of  steel  to  concrete,  then  n 
would  always  equal  -36  d.  It  will  be  advis- 
able, therefore,  to  use  the  above  formula  and 
find  what  proportion  n  will  be  of  d  if  the 
economical  amount  of  steel  is  used  as  an 
illustration  of  the  use  of  the  formula,  as 
follows  : — 

n   =  [v/wi2  r2  +2mr  —  mr]d 

m  —  15,  and  r   =  -00675,  therefore 

n   =  [v/152  x  -006752  +  2  x  15  x  -00675 

-  15  x  -00675] d 

n  =  [^225  x  -0000455625  +-2025  —10125]  d 
n   =  [x/ -0102515625  +  -2025  -  -10125J  d 

V/-2127515625  -  -10125] d 
n   =   -46125  -  -10125]<2 
n   =  -36  d 

A  great  number  of  figures  are  retained 
after  the  decimal  point  with  the  object  of 
showing  that  the  result  absolutely  agreed 
with  that  previously  given.  Formulae,  there- 
fore, are  available  for  finding  the  value  of 
n  in  all  cases,  namely  (1)  when  c  and  t  only 
are  known,  (2)  where  the  size  of  the  beam 
and  area  of  reinforcement  are  known  and  c 
and  t  are  unknown,  and  (3)  when  c  and  t  are 


unknown  and  the  area  of  the  steel  is  expressed 
as  a  proportion  of  the  area  of  concrete. 

Designing  Beam  for  Certain  Condi- 
tions.— As  a  final  example,  showing  the 
application  of  the  various  formulae,  a  case 
will  be  taken  where  it  is  required  to  design 
a  beam  to  carry  a  uniformly  distributed 
load  of  350  Ib.  per  foot  run,  in  addition 
to  its  own  weight,  over  a  span  of  24  ft. 
When  designed  the  stresses  in  the  steel  and 
concrete  will  be  checked  to  ascertain  whether 
they  are  in  accordance  with  the  permissible 
limits. 

It  will  here  be  seen  that  the  weight  of  the 
beam  itself  has  to  be  added  to  the  external 
load  before  the  bending  moment  can  be 
found,  and  as  the  size  is  not  yet  known,  an 
assumption  must  be  made  in  order  to  do 
this.  A  little  experience  will  enable  the 
designer  to  gauge  the  size  sufficiently  near 
the  actual  size  required  to  give  a  weight 
for  the  beam.  In  this  instance  the  depth  will 
be  taken  at  16  in.,  which  is  one  eighteenth 
of  the  span,  and  b  can  be  taken  at  10  in., 
which  is  just  over  -6  d.  To  the  amount  of 
d  2  in.  should  be  added  as  a  covering  for 
the  reinforcement,  giving  a  total  size  of 
18  in.  by  10  in.  The  weight  of  the  beam 
will  be  its  cubical  contents  multiplied  by 
150  Ib.  per  cub.  ft.  This  equals  24  ft.  x 
1-5  ft.  x  -63  ft.  'X  150  Ib.,  or  for  1-ft.  run 
=  1-5  ft.  x  -63  ft.  x  150  Ib.  =  141-75  Ib., 
say  142  Ib. 

The  total  load  to  be  carried  per  foot  run 
therefore  equals  350  +  142  =  492  Ib.    This, 
multiplied  by  the  span,  will  give  the  total 
load  on  the  beam,  thus  492  x  24  =  11,808 

Ib.     The  bending  moment  will  equal  -^-. 


BM  = 


11808  x  24  x  12 


B  M   =  425,088  in.-lb. 
The  moment  of  resistance  of  the  beam  must 
equal  425,088  in.-lb.,  and  the  depth  must  first 

3/ 

be  calculated  by  the  formula  d  =  A/ 


BM 

57  ' 


d  = 


425088 


d  =  ^/7457 
d   =  19-5  in. 

Take  6  at  -6  d  =  19-5  x  -6  =  11-7  in.     The 
area  of  steel  will  be  given  by  taking  the 
economical  ratio,  namely  -00675. 
At   =  -00675  b  d 
At   =  -00675  x  11-7  x  19-5 
At   =  1-54  sq.  in. 


THEORY    OF    REINFORCED    CONCRETE 


93 


If  three  bars,  |f  in.  in  diameter,  are  taken, 
this  will  give  an  area  of  1-55  sq.  in.,  which 
will  be  quite  suitable.  The  beam,  as  now 
designed,  has  an  effective  depth  of  19-5  in., 
with  a  breadth  of  11-7  in.  and  steel  reinforce- 
ment of  1-55  sq.  in.  Add  2  in.  to  the  depth 
for  covering  the  reinforcement,  and  make 
the  breadth  a  practical  size,  and  the  beam 
becomes  say  22  in.  by  12  in. 

Checking  Beam  after  Designing.— It 
is  now  required  to  check  the  stresses  in  this 
beam,  and  as  the  actual  size  of  the  beam  is 
now  available  it  will  be  advisable  to  work 
out  the  bending  moment  again,  because  the 
calculated  size  is  in  excess  of  that  assumed 
in  the  first  instance. 

Therefore  the  weight  of  the  beam  per  foot 
run  -  If  ft.  x  1  ft.  x  150  Ib.  =  275  Ib. 
Total  load  per  foot  run  =  350  +  275  =  625 
Ib.  The  total  load  to  be  carried  equals  the 
load  per  foot  run  multiplied  by  the  span  = 
625  x  24  =  15,000  Ib.  The  bending  moment 
wl  ,  „_,  15000x24x12 

=  -A-.  Therefore  B  M  =  ^ 

o  o 

=  540,000  in.-lb. 

The  position  of  the  neutral  axis  must  next 
be  found  by  the  formula 

/2  At  m  d        /At  m\2       Atm 

/2  x  1-55  x  15  x  19-5      /1-55  x  15\2 
n=V  ~W~  H      12      / 

1'55  x  lo 
12 


t  = 


BM 


t  = 


540000 


1-55 


t  = 


t  = 


540000 


1-55  x  17-18 
540000 


m.-lb. 


/906-75        /23-25X2 
=  V  12        '  \~12~) 


23-25 


12  \  12  /  12 

n  ==   V75-562  +  3-751  -  1-93 
n  =   v/79-313  -  1-93 
n  =  8-9  -  1-93 
n  =  6-97  in. 

The   stress  in  the   concrete  can  now  be 
found  by  the  formula 

2BM 

c  = 

b  n   > 


c  = 


c  = 


2  x  540000 


12  x  6-97  x 

1080000 
12  x  6-97  x  17-18 


1080000 
=  :=  c~-~-  7531b" 


This  is  too  high,  as  the  permissible  limit 
is  600  Ib. 
Now  check  the  stress  in  the  steel. 


This,  again,  is  much  too  high,  as  the  per- 
missible limit  is  16,000  Ib.  for  ordinary  mild 
steel,  and  even  for  stronger  steel  the  stress 
should  not  exceed  20,000  Ib. 

This  example  was  given  to  show  the  value 
of  checking  the  stresses  after  the  beam  has. 
been  designed,  when  the  size  has  to  be 
assumed,  and  consequently  an  error  of  judg- 
ment may  take  place,  especially  in  the  case 
of  those  inexperienced  in  design. 

The  next  step  is  to  see  in  what  manner  the 
beam  can  be  adjusted  to  bring  the  stresses 
within  the  limits.  The  weight  of  the  beam 
allowed  in  the  first  instance  was  insufficient, 
it  being  only  142  Ib.  per  foot  run,  whereas 
the  calculated  beam  was  275  Ib.  per  foot  run. 
Here,  however,  is  some  basis  upon  which  to 
assume  the  size,  and  if  the  depth  is  increased 
to  22  in.,  and  the  breadth  to  -6  d  =  say 
13-5  in.,  they  cannot  fail  to  be  near  the 
required  size. 

This  may  seem  a  great  deal  of  work  and 
calculation  in  connection  with  one  beam, 
but  it  must  be  borne  in  mind  that  the 
example  is  merely  given  to  illustrate  princi- 
ples and  familiarise  the  reader  with  the 
formulae  employed. 

Again,  in  a  building  where  the  plan  is 
symmetrical,  it  usually  happens  that  there 
are  several  beams  which  have  the  same  span 
and  carry  the  same  amount  of  load,  and  con- 
sequently when  the  calculations  for  one  beam 
are  complete,  many  beams  can  be  considered 
as  designed,  and  it  is  worth  some  little 
trouble  to  produce  economical  and  efficient 
design. 

The  reinforcement  might  be  increased  in 
this  example,  and  thus  the  strength  of  the 
beam  increased,  but  the  economical  per- 
centage would  not  then  be  employed. 

Of  course,  it  is  quite  simple  to  design  a 
member  which  is  perfect  and  suitable  for 
carrying  a  certain  load  as  given  in  various 
textbooks,  but  a  great  number  of  these 
examples  are  done  by  selecting  a  beam  with 
a  certain  ratio  of  reinforcement,  finding  the 


94 


REINFORCED    CONCRETE 


moment  of  resistance,  and  working  back- 
wards, as  it  were,  to  find  the  safe  load  per 
foot  run  over  a  certain  span.  This  method, 
of  course,  is  not  stated,  but  the  examples 
frequently  given  in  the  textbooks  are  based 
on  the  particulars  obtained,  and  thus  no 
difficulty  is  encountered. 

The  object  of  the  above  example  is  rather 
to  show  how  to  deal  with  a  case  where  the 
initial  calculation  is  found  to  be  incorrect, 
and  to  show  in  what  manner  these  may  be 
utilised  in  the  second  calculation.  It  will 
be  seen  that  in  the  first  instance  the  depth 
was  based  on  the  span,  as  the  bending 
moment  was  unknown,  and  it  goes  to  show 
how  essential  it  is  to  have  some  connection 
between  the  depth  and  the  bending  moment 
if  economical  design  is  to  result.  However, 
the  actual  bending  moment  to  be  resisted  is 
known  when  the  beam  had  an  effective  depth 
of  19-5  in.  and  a  breadth  of  12  in.,  namely, 
540,000  in.-lb.  Calculate  d  according  to 


this  as  a  guide  by  the  formula  d  = 


BM 

57  ' 


then  d  = 


540000 
57 


=  d  =  y  9473,  d  = 


21  in.  nearly.  It  must  be  realised,  however, 
that  the  fact  of  increasing  the  size  of  the 
beam  will  again  increase  the  weight  of  the 
beam,  and  therefore  the  bending  moment, 
and  an  allowance  must  be  made  for  this  ; 
and  for  this  reason  it  will  be  advisable  to 
take  d  at  22  in.  as  stated  above,  and  b  at 
•6  d  =  say  13-5  in. 

The  beam  can  now  be  worked  out  and 
again  checked  to  see  if  the  stresses  are  quite 
satisfactory.  The  size  of  the  beam  =  22  in. 
x  13-5  in.,  and  the  sectional  area  =  297 
sq.  in.  At  should  equal  -00675  b  d.  This 
equals  297  x  -00675  =  2-00475  sq.  in. 
The  area  of  a  round  rod  with  a  diameter 
of  ||  in.  =  -6903  sq.  in.,  and  3  rods  = 
2-0709  in.,  which  is  more  than  sufficient. 
Provision  must  be  made  for  covering  the 
bars  with  concrete,  and  if  2  in.  be  given,  the 
total  depth  becomes  22  +2  =  24  in. 
Having  now  designed  the  beam,  the  actual 
stresses  can  be  checked,  and  to  do  this  the 
actual  bending  moment  must  be  found  due 
to  the  given  load  to  be  carried,  and  the 
weight  of  the  beam  itself. 

The  latter  per  foot  run  =  2  x  1£  x  150  Ib. 
=  say  337  Ib.  The  total  load  per  foot  run 
=  350  +  337  =  687  Ib.  The  total  load  = 
load  per  foot  run  x  span  =  687  Ib.  x  24 


=    16,488   Ib.      The    bending    moment    = 

Mel  16488  x  24  x  12 

—r,    therefore    B  M    =  — ~ — 

=  593,568  in.-lb. 

The  position  of  the  neutral  axis  must  next 
be  found  by  the  formula 


2Atmd 


/£  ±±t 
n=\/—b—  + 


Lrw\2     _  At  m 
b   )  b 


/2  x  2-0709  x  15 

x22^ 

C1 

0709 

x!5> 

2 

/      13-5 

2-0709  x 

15 

( 

13- 

5   ; 

13-5 

/  1366-794       /31-0635\2       31-0635 

ss  »  /    .  _    _1_  I  _  I      _  _ 

V         13-5      ^V    13-5   /  13-5 


n  =  v/  101-24  +5-294  -  2-301 
n  =  10-32  -  2-301  =n  =  8-01  in. 

The  stress  in  the  concrete  can  now  be 
found  by  the  formula 

2BM 


b  n  x 


(-9 

2  x  593568 


c    — 


13-5x8-01 


1187136 


c  — 


13-5  x  8-01  x  19-33 
1187136 


2090-2 
c   =  567-9  Ib. 

It  will  be  advisable  to  check  the  stress  in 
the  steel  as  follows  : 

BM 


t   = 


A1  x 


t   = 


('-8 

593568 


2-0709  x 


t    = 


t  = 


593568 

2-0709  x  19-33 
593568 


40-03 
t  =  14,828  Ib. 

This  amount  is  well  below  the  permissible 
limit  of  16,000  Ib.  The  beam  as  designed 
will  therefore  have  a  total  depth  of  24  in., 
a  breadth  of  14  in.,  and  be  reinforced  with 
three  -J-g--in.  diameter  rods. 


THEORY    OF    REINFORCED    CONCRETE 


95 


Numerous  problems  and  examples  should 
be  worked  out  with  a  view  to  becoming  quite 
proficient  in  the  use  of  the  formulae,  and 
conversant  with  the  method  of  making 
assumptions  which  are  necessary"  in  some 
cases  to  form  a  basis  for  the  calculations, 
since,  although  the  principles  never  vary, 
the  method  of  procedure  is  likely  to  do  so 
under  different  circumstances. 

The  beam  would  also  be  required  to  be 
calculated  for  shear  and  adhesion,  but  these 
will  be  dealt  with  subsequently.  In  any 
examples  shown  up  to  the  present  any 
stress  other  than  that  due  to  simple  bending 
has  been  disregarded. 

CALCULATIONS   FOR   SLABS 

Slabs  can  be  dealt  with  in  a  similar  manner 
to  beams  as  regards  the  calculations  of 
strength,  but  the  value  of  the  bending 
moment  will  vary  according  to  the  propor- 
tion of  the  slab,  if  the  slab  is  supported  on 
all  four  sides ;  the  formula  for  finding  the 
depth  in  relation  to  the  bending  moment 
must  be  varied,  as  b  will  no  longer  be  equal 
to  -6  d,  but  will  always  equal  12  in.,  as  a 
strip  of  this  width  is  calculated.  In  the  case 
of  a  slab  supported  on  all  four  sides  and 
reinforced  in  two  directions,  the  weight  will 
be  distributed  on  all  four  supporting  edges, 
and  consequently  if  a  strip  of  the  slab  12  in. 
wide  is  taken  and  the  bending  moment  cal- 
culated, such  bending  moment  can  only  be 
due  to  the  reactions  which  occur  at  the  ends 
of  the  strip.  If  the  total  weight  upon  the 
slab  is  distributed  upon  all  four  edges,  it 
will  follow  that  the  reaction  at  any  given 
point  will  be  less  than  if  the  slab  were  sup- 
ported on  two  sides  only.  A  reduction  in 
the  bending  moment  must  therefore  be 
made  to  allow  for  this.  If  the  slab  is  per- 
fectly square,  all  four  edges  will  carry  an 
equal  amount,  and  the  reaction  at  any  one 
side  will  be  one-half  the  amount  that  would 
occur  if  the  slab  were  supported  on  two  sides 
only.  If  the  reaction  is  only  half  the 
amount,  then  the  bending  moment  will  be 
reduced  by  a  similar  factor. 

The  bending  moment  at  the  centre  of  a 
beam  or  slab  supported  at  the  ends  is  equal 

w  I 
to  -£-,  and  therefore  the  bending  moment 

at  the  centre  of  a  square  slab,  supported  on 

wl 

all  four  edges,  is  equal  to  -TTT.    Again,  the 

bending  moment  at  the  centre  of  a  beam  or 
slab  which  is  fixed  at  the  ends  is  equal  to 


-,  - 


,  while  with  a  square  slab  fixed  on  all  four 


.TIT  W 

edges,  the  bending  moment  is  equal  to  -^ 

These  formulae,  of  course,  apply  to  cases 
when  the  load  is  uniformly  distributed. 
This  advantage  may  be  considered  as  occur- 
ring when  the  length  of  the  slab  does  not  ex- 
ceed twice  the  breadth  ;  when  the  length 
exceeds  this  proportion,  the  slab  is  considered 
as  one  which  is  supported  or  fixed  on  two 
edges  only.  The  advantages  will  vary  be- 
tween these  two  cases,  namely,  a  square 
slab,  and  a  slab  where  the  length  is  twice 
the  breadth,  and  the  variation  will  depend 
upon  the  ratio  of  the  length  to  breadth. 

The  draft  regulations  of  the  London  County 
Council  give  formulae  showing  the  allowance 
that  may  be  made  as  follows  :  — 

w  =  weight  on  slab  (total  distributed 
weight,  including  its  own  weight). 

b   =  breadth  of  slab. 

I    =  length  of  slab. 


Bending 

Condi- 

B.M. 

B.M. 

moment  at  any 

tion 

for 

for 

given 

of 

shorter 

longer 

cross  section 

supports 

span 

span 

wb         1 

wl         1 

At  centre  of  span 

Free 

8     l  +  (b/l)* 

8     l  +  (l/b)* 

At  end  and 
centre  of  span 

Par- 
tially 
fixed 

wb         1 

wl         1 

12    l  +  (6/J)4 

12  1  +  (*/&*) 

Bending  Moment  at  Centre  of  Slab. 

—  It  will  be  advisable  to  give  an  example  to 
illustrate  the  use  of  these  formulae  and  show 
the  decrease  in  the  bending  moment  which 
is  given  by  same.  Assume  a  slab  8  ft.  by 
6  ft.,  which  is  supported  on  all  four  sides, 
and  carries  a  uniformly  distributed  load  of 
250  Ib.  per  sq.  ft.,  and  let  it  be  required  to 
calculate  the  bending  moment  at  the  centre 
of  the  span  in  both  directions,  namely,  (1) 
by  considering  a  portion  1  ft.  wide  and 
6  ft.  long,  and  (2)  by  considering  a  portion 
1  ft.  wide  and  8  ft.  long. 

By  referring  to  the  table  above,  it  will  be 
seen  that  the  formula  for  the  shorter  span 

is  X  w  =  the  total 


and  this  will  equal  the  area  x  by  the  weight 
per  ft.  super.  Therefore  w  =  6  ft.  x  1  ft.  x 
250  Ib.  ;  w  =  1,500  Ib.  ;  6  =  the  breadth 


96 


REINFORCED    CONCRETE 


of  the  slab,  which  is  6  ft. ;    I  =  the  length 
of  the  slab,  which  is  8  ft.     Hence, 
t0&        ^L 
8    X  1- 


1500  Ib.  x  72  in.  v        1 

;    . X         — 


1 


=  13500  x 


=  10,258  in.-lb. 


1-316 

The  allowance  made  in  this  instance  is 
sufficient  to  reduce  the  bending  moment 
from  13,500  in.-lb.  to  10,258  in.-lb. 

Now  calculate  the  bending  moment  for 
the  longer  span,  according  to  the  formula 
wl  1 


In  this  case  w  =  8  ft.   x   1  ft.   x   250  = 
2,000  Ib.  Hence,  B  M  =  — 

O  i 


B  M  =  24000  x 


1 


-  24000  x 


4-16 


1  +  3-16 
B  M  =  5,769  in.-lb. 

The  reduction  is  considerably  more  in  this 
instance,  due,  of  course,  to  the  fact  that 
the  edges  of  the  short  span  will  carry  a 
greater  portion  of  the  load  than  the 
edges  of  the  long  span,  and  as  the  reactions 
will  be  less,  so  the  bending  moment  will 
be  reduced. 

With  regard  to  the  thickness  of  the  slab, 
this  should  be  calculated  according  to  the 
bending  moment,  and  the  following  formula, 
with  its  construction,  is  given  for  this  pur- 
pose. It  has  already  been  seen  by  previous 

(n\ 
d  ~~  Q  )' 
./ 
and  that  it  is  possible  to  substitute  definite 

values  for  certain  symbols  as  follows  :  b 
=  12  in.,  c  =  600  Ib.,  and  n  =  -36  d,  thus 
reducing  the  formula  to 

2  B  M  =  12  x  600  x  -36  d  x  (d  - 


2  B  M  =  12  x  600  x  -36  d  x  -88  d 

2  B  M  =  7200  x  -3168  d2 

2  B  M  =  2280-96  d2  =  B  M  =  1140-48  d2 


/  BM 

=  V  1140-' 


M 

1140-48 
It  will  be   quite  near  enough  to  use  the 

/B~M 
lormula  as  a  ==  /y/  VMA'  ™e  omission  of 

the  decimal  figures  making  no  appreciable 
difference  in  the  result.  In  the  case  of  a  slab 
which  is  supported  on  all  four  edges,  it  has 
been  shown  that  it  is  necessary  to  calculate 
the  bending  moment  for  both  spans,  and  as 
the  shorter  span  will  give  the  greater  bending 


moment,  as  shown  by  the  example  above,  the 
depth  should  be  calculated  to  suit  this,  and 
as  this  will  be  in  excess  of  that  required  for 
the  longer  span,  the  longitudinal  reinforce- 
ment required  will  be  diminished  accord- 
ingly. Again,  it  will  obviously  be  an  advan- 
tage, theoretically,  to  place  the  reinforce- 
ment across  the  slab  below  that  parallel 
with  the  length  of  the  slab,  in  order  to  obtain 
the  maximum  effective  depth  to  resist  the 
greater  bending  moment. 

If  the  depth  is  found  by  calculation  to  be 
less  than  3|  in.,  it  should  be  increased  to  this 
amount  for  practical  purposes,  as  any  thick- 
ness under  this  is  likely  to  be  unreliable. 
The  least  diameter  or  thickness  of  the  main 
reinforcement  should  not  be  less  than  \  in., 
and  any  other  reinforcement  should  not  be 
less  than  \  in.  in  diameter.  The  maximum 
distance  between  the  main  tensile  reinforce- 
ment should  not  be  greater  than  12  in.,  or 
less  than  1  in.,  and  the  spacing  of  the  rein- 
forcement, when  the  load  is  uniformly  dis- 
tributed, may  be  gradually  increased  from 
the  middle  third  to  the  outer  edges,  if  the 
slabs  of  the  bars  are  kept  of  a  uniform  size, 
provided  that  the  spacing  at  the  outer  edges 
be  not  greater  than  three  times  the  spacing 
at  the  centre  of  the  slab. 

Calculating  Slab  Supported  on  Four 
Sides.  —  As  an  example  of  slab  design,  let 
it  be  required  to  calculate  the  thickness  and 
reinforcement  for  a  slab  12  ft.  by  8  ft.,  sup- 
ported on  all  four  sides  to  carry  a  load  of 
275  Ib.  per  square  foot  in  addition  to  its 
own  weight.  The  stresses  in  the  concrete  and 
steel  not  to  exceed  600  Ib.  and  16,000  Ib. 
respectively. 

The  weight  of  the  slab  itself  will  require 
to  be  found  in  the  first  instance,  as  this 
must  be  added  to  the  external  load  in  order 
to  ascertain  the  bending  moment.  Assume 
a  total  thickness  of  6  in.  for  this  purpose, 
and  the  weight  per  square  foot  will  then  be 
1  ft.  x  1  ft.  x  -5  ft.  x  150  Ib.  =  75  Ib. 
The  total  load  per  square  foot  is  equal  to 
275  Ib.  +  75  Ib.  =  350  Ib.  It  is  now  re- 
quired to  calculate  a  portion  of  the  slab, 
8  ft.  long  and  1  ft.  wide,  and  the  area  will 
equal  8  sq.  ft.  The  total  load  on  this  por- 
tion will  be  the  area  multiplied  by  the  weight 
per  square  foot  ;  thus  8  x  350  =  2,800  Ib. 
total  load.  The  bending  moment  can  now 
be  calculated,  bearing  in  mind  that  the  slab  is 
supported  on  all  four  sides,  and  the  formula 


i  /A/\4  must  be  employed.    There- 


THEORY    OF    REINFORCED    CONCRETE 


97 


fore     the     bending     moment 
2800  x  96  in.  1 


will     equal 


1  + 


BM  =  33600  x 


1-197 


B  M  =  28,070  Ib. 

Having  now  obtained  the  bending  moment, 
the  depth  can  be  calculated  from  it  by  the 
formula  previously  given,  namely : 

/BM 
=  V  i 


d 


1140 


d  =  V2.4'8 
d   =  4-9  in. 

To  use  the  economical  percentage  of  rein- 
forcement, the  sectional  area  must  be  made 
equal  to  -00675  6  d,  and  this,  therefore,  will 
be  -00675  x  12  x  4-9  =  -3969  sq.  in.  The 
area  of  one  £-in.  round  rod  equals  -1963 
sq.  in.,  and  two  rods  will  give  a  sectional 
area  of  -3926  sq.  in.  This  is  such  a  small 
fraction  under  the  required  amount  that 
it  can  be  used,  and  as  d  was  found  to  be 
equal  to  4-9  in.,  this  may  be  increased  to  5  in. 
for  practical  purposes.  As  there  will  be 
two  ^-in.  rods  for  every  foot  in  width,  they 
will  be  spaced  at  6-in.  centres,  and  this  will 
be  quite  satisfactory.  By  adding  1  in.  of 
concrete  to  the  effective  depth  of  5  in.,  in 
order  to  afford  covering  for  the  reinforce- 
ment, the  total  depth  will  be  equal  to  6  in., 
which  exactly  agrees  with  the  assumption 
made  in  the  first  instance. 

It  will  now  be  necessary  to  check  the 
stresses  in  the  steel  and  concrete  to  see  that 
they  do  not  exceed  the  permissible  limit. 

The  weight  of  the  slab  having  been  cor- 
rectly assumed,  it  will  not  be  necessary  to 
recalculate  the  bending  moment,  as  in  the 
case  of  the  beam  designed  in  the  previous 
example,  and  the  position  of  the  neutral 
axis  is  the  only  unknown  factor.  This  will  be 
found  by  using  the  formula  already  given  : — 

/2  Atmd       /At  m\  2      A<  m 
V     ~b        +\~b ~)    ~~b~ 
/2  x  -3926  x  15  x  5       /-3926  x  15\2 

=v      IT-    -+(~^-) 

•3926  x  15 

=  yx 4-9075  +  (-49075)2  -  -49075 
=  y/ 4-9075  +:2408  -  -49075 
-  v/ 54483  -  -49075 
-  2-26  -  -49075 
=  1-769 

7 


It  is  interesting  to  compare  this  value  for 
n  with  that  which  would  be  given  by  taking 
same  as  -36  d,  such  as  would  be  the  case  if 
the  stresses  were  known  to  be  600  and 
16,000  Ib.  respectively ;  then  it  would  equal 
•36  x  5  in.  =  1-8,  which  is  very  near. 

To  check  the  stress  in  the  concrete  will 
be  the  next  step. 

2BM 


c   = 


bn 


x  I  a  —  7: 


c   = 


2  x  28070 


12  x  1-769  x 
56140 


C    =   , r~ 


_  56140 
*  C         93-4 


12x1-769x4-4 
c   =  601  Ib. 

This  can  be  considered  as  satisfactory,  and 
the  stress  on  the  steel  be  checked  by  the 
formula  : 

BM 

t  =  — 


t   = 


At  x  Id  — 

28070 


28070 
1-727 


•3926  x  4-4 
t   =  16,253  Ib. 

This  is  slightly  in  excess  of  the  permissible 
limit,  and  to  overcome  this  the  spacing  of 
the  bars  may  be  slightly  reduced,  say  from 
6-in.  to  5-in.  centres.  The  student  is  advised 
to  do  this,  and  again  check  the  value  of  t, 
as  these  examples  are  merely  given  for  the 
purposes  of  illustration,  and  constant  prac- 
tice and  experience  will  be  the  best  asset  of  the 
designer  when  he  has  once  mastered  the  prin- 
ciples and  formulae  which  govern  his  design. 

It  is  necessary  now  to  find  the  longitudinal 
reinforcement  that  will  be  necessary,  and  for 
this  purpose  the  bending  moment  must  be 
worked  out  for  the  longer  span.  As  shown 
above,  the  total  load  per  square  foot  = 
350  Ib.,  and  the  load  on  a  strip  1  ft.  wide 
and  12  ft.  long  will  therefore  equal  350  x  12 
=  4,200  Ib.  The  bending  moment  will  be 
found  by  the  formula  for  the  longer  span  , 

i.-  -u  •    wl  l 

which  is  -tr  x  — 


BM  = 


4,200  x  144  in. 


1  +1-= 


BM  =  75COO  x 
BM 


1  +  5-06 


75600 


6-06 


=  12,475  in.-lb. 


98 


REINFORCED    CONCRETE 


It  will  not  be  necessary  in  this  instance  to 
calculate  the  depth,  as  this  is  given  by  the 
calculations  for  the  shorter  span,  when  it 
was  found  to  be  5  in.,  and  as  the  longitudinal 
reinforcement  will  be  placed  on  the  top  of 
the  cross-rods,  d  will  be  reduced  by  the  dis- 
tance between  the  centres  of  the  two  sets 
of  rods,  which  will  be  £  in.  if  the  longitudinal 
rods  are  of  the  same  diameter.  It  will  be 
better  to  keep  them  the  same  size  if  pos- 
sible and  space  them  farther  apart.  The 
depth  will  then  be  5  in.  —  -5  in.,  which 
equals  4-5  in.  In  order  to  make  quite  sure 
that  this  is  sufficient,  d  may  be  calculated 
according  to  the  bending  moment  given  and 
the  results  compared.  As  before  : 

: 


1UO 


12470 
1140 


d  = 


d  =    VlO  =  3-1  in. 

This  is  less  than  the  value  already  obtained, 
and  the  amount  of  4-5  in.  may  be  worked  to, 
which  will  have  the  effect  of  reducing  the 
amount  of  reinforcement  required,  as  the 
lever  arm  of  the  internal  resistances  will  be 
increased.  This  being  so,  seek  for  some 
method  of  deducing  the  value  of  At  when 
d  is  given  and  is  in  excess  of  that  required 
by  calculating  same  from  the  bending 
moment.  The  bending  moment  is  known, 
the  depth  is  known,  and  n  should  be  equal 
to  -36  d  if  the  stresses  are  to  be  satisfactory, 
and  the  formula  for  finding  t,  which  is 

BM 

t  =  -      —/  —    —  r,  can  be  used.     If,  how- 


At  x  (d  - 

ever,  a  value  is  given  to  t  of  16,000  Ib.,  then 
there  is  only  one  unknown  factor,  namely 
At,  and  this  can  be  found  as  follows  : — 


16000  = 


A,  = 


-36  d 


12475 


16000  x  (d  -  -12  d) 

12475 
16000  x  -88  d 

12475 


12475 
63360 


16000  x -88  x  4-5 
At  =  -1968  sq.  in. 

The  area  of  one  £-in.  diameter  bar  equals 
•1963  sq.  in.,  and  therefore  this  size  can  be 
employed  and  spaced  at  12-in.  centres, 
which  is  the  maximum  permissible  limit  for 
tensional  bars. 


Checking  the  Slab. — It  will  now  be 
necessary  to  check  the  stresses  in  the  con- 
crete and  steel,  and  in  order  to  do  this  the 
position  of  the  neutral  axis  must  first  be 
found  as  follows  :  — 


2  At  m  d 


/A^mY- 
I    b~) 


At  m 


/2x  -1963x15x4-5        /-1963xl5 
:  V  ~i2~~  '  \ 


12 


•1963  x  15 


12 


n  =    v/2-2083  +  (-245S)2  -  -2453 
n  =    x/2-2684  -  -2453 
n  =  1-5  —  -2453  =  n  =  1-254  in. 
For  checking  the  stress  in  the  concrete  : 
2BM 

C    =     -  —T- 

bn  x  (d  — 


c  = 


2  x  12475 


12  x  1-254  x 
24950 


c  = 


24950 
61425 


12  x  1-254  x  4-082 
c  =  406  Ib. 

This  is  well  below  the  permissible  limit, 
but  t  should  be  checked,  and  this  will  pro- 
bably be  very  near  the  amount  of  16,000  Ib. 

BM 

t  = 


-(•-9 


12475 


t  = 


•1963  x 
12475 


•1963  x  4-082 
_  12475 
: 


t  =  15,568  Ib. 

This  amount  is  not  very  much  below  the 
permissible  value,  and  it  will  not  be  advis- 
able, therefore,  to  decrease  the  reinforce- 
ment in  any  way,  and  the  slab  can  be  con- 
sidered as  satisfactorily  designed.  It  will 
be  impossible  to  stress  the  concrete  to  its 
full  extent  without  overstressing  the  steel, 
as  the  depth,  which  influences  the  amount, 
of  concrete  is  in  excess  of  that  theoretically 
required,  as  previously  explained.  Figs.  121 
and  122  show  how  the  dimensions  are  taken. 
It  is  interesting  to  note  what  the  result 
would  be  if  the  area  of  the  reinforcement 
had  been  based  on  the  theoretical  depth 
instead  of  taking  advantage  of  the  depth 
given,  and  for  the  guidance  of  the  student 


THEORY    OF    REINFORCED    CONCRETE 


99 


this  will  be  worked  out.  It  lias  been  seen 
that  this  depth  was  3-1  in.,  and  the  theoretical 
area  of  the  concrete  in  section  therefore 
equalled  3-1  x  12  in.  -  37-2  sq.  in.  If  At 
was  made  equal  to  -00675  b  d,  then  it  would 
be  37-2  x  -00675  =  -2511  sq.  in.  This  is 


6'l  d=S' 

/LONGITUDINAL  BARS 


Fig.    121. — Section  Through  Gross  Rods 

greatly  in  excess  of  the  amount  actually 
required,  and  as  the  stresses  as  shown  above 
are  even  now  below  the  permissible  limits, 
it  is  obvious  that  they  would  be  far  more 
so  in  this  case,  and  the  design  would  not  be 
economical. 

In  the  case  of  slabs  which  are  fixed,  the 
bending  moment  at  the  ends  of  the  spans 
must  also  be  calculated,  and  reinforcement 
provided  on  the  upper  surface  to  take  the 
tension  which  will  occur.  This  is  effected 
by  bending  up  some  or  all  of  the  bars  in 
the  lower  surface  when  they  are  no  longer 
required  in  the  latter  position ;  or  short 
bars  may  be  placed  on  the  upper  surface 
and  carried  for  the  necessary  distance  into 
the  slab.  The  turning  up  of  the  lower  bars 
will  assist  in  the  resistance  to  shear,  and 
this  will  be  more  fully  explained  and  illus- 


i>=/2"- 


2,  PODS 

^CgO 

jr-  —  


12' 


Fig.   122. — Section  Through  Longitudinal    Rods 

trated  in  the  notes  dealing  with  this  por- 
tion of  the  subject.  It  must  be  borne  in 
mind,  however,  that  it  will  not  be  sufficient 
to  calculate  the  bending  moment  in  the 
centre  of  the  span  only,  and  although  the 
same  theory  and  principles  will  apply  in 
both  cases,  the  student  is  advised  to  take 
an  example  of  a  fixed  slab  and  calculate 


the  reinforcement  necessary  at  the  various 
points  for  the  purposes  of  practice. 

BEAMS      WITH      DOUBLE       REINFORCE- 
MENT 

With  regard  to  the  beams  and  slabs 
described  up  to  the  present,  no  restrictions 
have  been  placed  on  the  size  of  the  members, 
and  consequently  the  depth  in  some  in- 
stances has  been  such  as  would  cause  great 
inconvenience  in  buildings  of  limited  height. 
To  allow  of  beams  of  less  depth  being  em- 
ployed, it  is  necessary  to  introduce  double 
reinforcement,  which  consists  of  two  sets  of 
rods,  one  of  these  taking  the  tension  as 
before,  and  the  other  taking  a  certain  pro- 
portion of  the  compression  which  the  reduced 
quantity  of  concret3  is  unable  to  resist. 
Another  consideration  is  the  weight  of  the 
beam.  In  the  case  of  long  spans,  a  beam 
with  single  reinforcement  may  be  so  heavy 
that  its  weight  is  greater  than  the  external 
load  to  be  carried  ;  the  introduction  of  com- 
pressional  reinforcement  reduces  the  size  of 
the  beam  necessary  and  thus  reduces  the 
total  load  to  be  carried.  This  is  an  impor- 
tant consideration,  as  not  only  is  the  bending 
moment  on  the  beam  reduced,  but  so  also 
is  the  load  to  be  carried  by  the  supports 
and  the  foundations. 

The  procedure  in  the  design  and  calcu- 
lations is  somewhat  different  from  that 
employed  in  the  case  of  beams  with  single 
reinforcement,  as  with  the  latter  it  is  neces- 
sary to  begin  with  an  assumption  regarding 
the  size  of  the  beam  required,  in  order  to 
calculate  the  bending  moment ;  but  now  a 
definite  size  can  be  fixed  for  the  beam  at  the 
outset  and  suitable  reinforcement  inserted. 
This  size  should  be  made  as  near  the  econo- 
mical section  as  the  circumstances  will  per- 
mit, if  there  is  any  choice  in  the  matter. 
As  the  size  will  thus  be  available  at  the  out- 
set, the  weight  of  the  beam  can  be  found  and 
the  bending  moment  calculated.  This  bend- 
ing moment  will  be  the  one  to  be  resisted, 
and  there  will  be  no  variation  or  re-calcula- 
tion on  this  point. 

The  next  step  will  be  to  consider  the 
strength  of  the  beam  according  to  the  fixed 
size  by  adding  the  economic  proportion  of 
einforcement,  and  considering  it  as  a  single 
reinforced  beam,  and  then  calculating  the 
bending  moment  that  such  a  beam  will 
resist.  It  will  necessarily  be  less  than  the 
actual  bending  moment  to  be  resisted,  and 
the  difference  between  the  two  will  give  the 


100 


REINFORCED    CONCRETE 


excess  that  has  to  be  taken  by  the  com- 
pressional  reinforcement  and  the  extra  ten- 
sional  bars  that  will  be  required  over  and 
above  the  economical  percentage. 

Figs.  123  and  124  will  be  some  guide  as 
to  the  disposition  of  the  reinforcement,  and 
the  meaning  of  the  symbols  employed. 

It  is  necessary  to  introduce  a  certain 
number  of  new  formulae,  owing  to  the  new 
factor  in  the  compressional  resistance,  and 
these  can  be  explained  in  the  simplest 
manner  by  taking  an  example  and  working 
it  out  step  by  step  with  the  necessary 
explanation  as  it  becomes  due. 

Designing  Beam  with  Double  Rein- 
forcement.— A  beam  is  required  to  carry 
a  uniformly  distributed  load  of  10  tons  in 


covering  the  reinforcement,  and  this  can  be 
taken  at  2  in.,  thus  giving  an  effective  depth 
of  20  in.  and  a  breadth  of  12  in. 

The  economical  amount  of  reinforcement 
will  be  found  by  multiplying  the  effective 
sectional  area  of  the  concrete  by  -00675, 
thus  20  x  12  x  -00675  =  1-62  sq.  in.  The 
moment  of  resistance  of  this  beam  will  be 
equal  to  the  sectional  area  of  the  steel  multi- 
plied by  the  safe  stress  per  square  inch 
multiplied  by  the  lever  arm  of  the  internal 

/          n 
resistance  expressed  as  At  x  t  x  (d  — 

and  the  bending  moment  will  equal  the 
moment  of  resistance,  therefore 


BM    =  At 


d 


x 


•-  I 

n 


-A 


d-ff 


Fig.  123  Fig.  124 

Figs.   123  and  124. — Double  Reinforcement  in  Beams 


addition  to  its  own  weight  over  a  span  of 
16  ft.,  while  the  over-all  size  is  limited  to 
22  in.  by  12  in.  The  actual  weight  of  the 
beam  must  first  be  calculated  in  order  to 
find  the  total  load.  This  will  equal  1|-  x  1 
x  150  lb.  x  16  ft.  =  4,400  lb.,  and  the  total 
load  =  10  x  2240  +  4400  =  26,800  lb.  The 

w  I 
bending  moment  will  equal  -?r   =  B  M   = 

26800  x  16  x  12 

— g—         -  =  B  M  =   643,200  in.-lb. 

The  actual  bending  moment  to  be  resisted 
has  thus  been  obtained,  and  it  is  now  re- 
quired to  ascertain  the  moment  of  resist- 
ance of  the  specified  beam  if  same  is  rein- 
forced in  the  tensional  area  only,  and  see 
how  far  it  is  deficient.  As  the  over-all  depth 
is  22  in.,  an  allowance  must  be  made  for 


To  find  the  value  of  n,  take  this  as  -36  d 
from  previous  deductions,  therefore  n  = 
•36  x  20  =  7-2  in.,  and 

BM  =  1-62  x  leOOO  x 

B  M   =  1-62  x  16000  x  17-6 

B  M  =  =  456192  in.-lb. 
The  actual  bending  moment,  however, 
equals  643,200,  and  the  excess  which  has  to 
be  provided  for  by  extra  tensional  reinforce- 
ment, and  compressional  reinforcement  = 
643200  -  456192  =  187008. 

It  may  assist  the  student  if  the  moment 
of  resistance  is  worked  out  with  regard  to 
the  concrete,  in  order  to  show  that  the  same 
result  is  obtained  and  that  the  compressional 
reinforcement  is  necessary.  Considering  the 
concrete  as  taking  600  lb.  per  sq.  in.,  this 


THEORY    OF   REINFORCED    CONCRETE 


101' 


C    X 


x    n 


=  25920   x 


gives  MR    = 

600  x  12  x  7-2 

~^T  "  \~"         3 

17-6  =  456192  in.-lb.  as  before.  It  is 
therefore  obvious  that  the  compressional 
reinforcement  will  be  required  to  take  the 
excess  of  187008  in.-lb. 

The  position  of  this  compressional  rein- 
forcement must  now  be  settled  in  order  to 
determine  the  lever  arm  between  the  two 
sets  of  reinforcement.  It  is  advisable  to 
place  this  as  far  as  possible  from  the  neutral 
axis  while  obtaining  a  covering  of  concrete, 
and  this  will  be  given  if  it  is  situated  at  a 
distance  of  2  in.  from  the  upper  surface,  as 


=      as  A/  = 


N 

20' 


12' 


r* 


12' 8 


Fig.    125. — Designing  Beam  Having  Double 
Reinforcement 


shown  in  Fig.  125.  There  are  thus  two  sets 
of  reinforcement,  one  situated  at  a  distance 
of  5-2  in.  from  the  neutral  axis  and  the 
other  at  a  distance  of  12-8  in.  from  the  same 
point. 

The  distance  from  the  upper  surface  to 
the  centre  of  the  compressional  reinforce- 
ment is  expressed  by  the  symbol  dc,  and 
the  distance  apart  or  lever  arm  of  the  two 
sets  of  bars  will  be  d  —  dc.  Considering, 
first,  the  additional  tensile  reinforcement 
required,  the  limiting  stress,  which  is 
16,000  Ib.  per  sq.  in.,  is  known,  and  also 
the  lever  arm,  which  is  d  —  dc.  If,  there- 
fore, the  excess  bending  moment  is  divided 
by  t,  x  (d  —  dc),  the  amount  of  steel  neces- 
sary will  be  found.  This  may  be  expressed 


Excess  B  M 
t  x  (d  -  d~Y 


Working  this  out 


gives  — 

A/   = 

A/   _ 

187003 

16000  x(20 
187003 

-2) 

/ 

t    — 

288000   ~  A 

•' 

This  additional  reinforcement  must  be 
added  to  the  amount  already  calculated  for 
in  the  tensional  area  amounting  to  1-62  sq.  in. 
making  a  total  of  2-269  sq.  in. 

The  value  of  the  compressional  resistance 
must  also  equal  the  excess  bending  moment, 
and  if  the  area  of  the  steel  in  compression 
be  expressed  by  the  symbol  Ac  and  the 
intensity  of  the  stress  by  the  symbol  c,, 
then  Ac  x  cs  x  (d  —  dc)  =  excess  B  M  and 

Excess  B  M 

c    ' ''   c —     (d  —  d  Y  value  of  c.,.  will 

require  to  be  found  before  proceeding  further, 
as  although  the  stress  intensity  could  safely 
be  16,000  Ib.  per  sq.  in.  in  compression  as 
in  tension,  it  will  not  be  possible  to  develop 
this  amount  on  account  of  the  steel  being 
so  much  nearer  the  neutral  axis  in  the  com- 
pression area  than  in  the  tension  area.  The 
stress  varies  directly  as  the  distance  from 
the  neutral  axis,  and  therefore,  if  the  com- 
pression steel  is  placed  twice  as  near  to  the 
axis  as  the  tension  steel,  then  it  can  only 
develop  one-half  the  stress  taken  by  the 
latter.  In  other  words,  the  stress  on  the 
compression  steel  multiplied  by  the  distance 
of  the  tension  steel  from  the  axis  must  equal 
the  stress  in  the  tension  steel  multiplied  by 
the  distance  of  the  compression  steel  from 
the  axis.  Expressed  by  symbols,  this  reads 
Cg  x  (d  —  n)  =tx  (n  —  dc),  therefore  o  = 

t  x  (n  -  dc) 
— -j-^ .     In  this  example — 

16000  x  (7-2  -  2) 


c*    = 


20  -  7-2 


832000 


12-8 

c,    =  6,500  Ib. 

Again,  this  may  be  shown  in  another 
way,  as  when  steel  and  concrete  are  acting 
together  in  compression  the  stress  in  the 
steel  can  only  reach  m  times  that  in  the 
surrounding  concrete,  as  it  is  only  under 
these  conditions  that  they  can  act  together. 
The  stress  in  the  concrete  at  the  extreme 
upper  edge  is  equal  to  600  Ib.,  and  conse- 
quently at  a  distance  of  dc  from  the  point 
it  will  be  less,  as  it  is  closer  to  the  neutral 


102 


REINFORCED    CONCRETE 


cxn-  d 


axis.     The  stress  will  be  equal  to 

which  expresses  proportion  of  stress  at  this 
point.     The  value  of  cs  as  above  will  equal 
m  times  this  amount,  and  therefore — 
m  x  c  x  (n  —  dt) 

15  x  600  x  (7-2  -  2) 

7-2 
cs    =  6,500  lb.,  as  before. 

Excess  B  M 

The  formula  Ar  =  -    —TJ j-\  can  now 

cs  x  (d  -  dc) 

be  worked  out  and  the  compressional  steel 
found,  thus  : 

187008 
A'   ==  6500  x  (20  -  2) 

187008 

A'   ~'~~  llTOOO 
Ac   =  1-598  sq.  in. 

The  beam  as  designed,  therefore,  has  an 
over-all  size  of  22  in.  by  12  in.,  with  an 
effective  depth  of  20  in.,  and  the  reinforce- 
ment consists  of  2-269  sq.  in.  in  the  tensional 
area  and  1-598  sq.  in.  in  the  compressional 
area. 

BEAMS     AND     SLABS     WITH      DOUBLE 
REINFORCEMENT 

In  checking  the  stresses  in  a  beam  already 
designed  and  containing  double  reinforce- 
ment, it  is  natural  that  the  formulae  should 
be  a  little  more  complicated  than  in  the 
case  of  single  reinforced  beams,  as  there  are 
more  factors  to  consider  in  the  resistance. 
In  the  case  of  the  stress  on  the  concrete 
there  is  some  portion  of  the  compression 
taken  by  the  steel  in  the  upper  part  of  the 
beam,  and  this,  obviously,  must  be  con- 
sidered in  arriving  at  the  value  of  c.  It  will 
be  necessary  to  see  how  the  formula  for 
finding  c  is  constructed.  As  a  basis  for 
the  reasoning,  there  is  the  theory  already 
evolved,  namely : 
cbn  I  n\ 
— qr~  x  ( d  —  Q  I  and  Ac  x  cs  x  (d  -  -  d.) 

Zi  \  O  / 

together  equal  B  M.  This  can  be  stated  as 
follows  : — 

2  B  M  =  c  b  n  (d  -  * )  +  2  Ac.  x  c,  x  (d-  df) 

\         6' 

The  factor  cs  will  be  unknown  until  the 
compression  stress  on  the  steel  is  found,  but 
its  equivalent  value,  as  previously  shown, 

c  x  m  x  n  —  dc 

is    -  — ,  and  this  can  be   sub- 

n 

stituted,  giving  : 


2  B  M  =  c  6  w  x 


M 


+  2  Ac  c  m  x 


Therefore  : 
2EM. 


=  clbn  x  (d  -    ^  +  2  Ac 

«•'-«'] 


m    x 


and  c  will  be  found  by  the  formula  : 

c  = 

_  2BM  _ 

b  n  x  (  d  -  1  j  +  2  Ac  m  x  (  -    —  CJ  x   (d-  dc) 

This  formula  appears  to  be  a  very  large  one, 
but  it  is  perfectly  simple,  and  will  very 
quickly  become  familiar  to  the  student, 
who,  however,  will  be  unable  to  make  use 
of  the  equation  until  the  value  of  n  is  known  ; 
and  it  will  be  necessary  to  deduce  a  formula 
for  this,  based  on  the  relative  values  of  the 
concrete,  steel  in  compression,  and  steel  in 
tension. 

It  is  known  that  the  total  compression  and 

cbn 
tension  are  equal,  and  therefore  -~  -  +  Ac 

x  cs  =  t  At,  or  c  b  n  +  2  Ac  x  cs  = 
2tAt. 

We  do  not  know  the  values  of  t,  c,  or  cs, 
but  can  replace  them  with  equivalents  as  be- 

t          m  x  d  —  n 

,  as  shown  by 
n 

c  m  (n  —  dc)} 


fore  ;  thus  -    = 


previous  reasoning,  and  cs  = 


cs        m 

OI  J  —  _ 

c  n 


—  d) 
—ci 


x  cs  =  2  t  At,    and    if    we    divide    both 
sides   of   the    equation  by  c  we   shall   get 

2  Ac  x  c,        2tAt 
bn  +  -  ~  ,  or  6  M  +  2  AS  x 


cs. 


t 


t  C- 

By  substituting  the  values  of  -  and   -  as 

C  C 

given  above,  we  can  remove  the  unknown 
factors  and  obtain  the  following  :  — 

2  Ac  m  (n  —  dc)          2Atm(d—n) 

bn  +      ~Y~  ~^~ 

Multiply  both  sides  of  the  equation  by  n  and 
we  have 

b  n2  +  2  Ac  m  (n  —  dc)  =  2  AC  m  (d  —  n). 
We  have  now  got  n  on  both  sides  of  the 
equation,  and  this  must  be  simplified  as 
follows  :  — 


THEORY    OF    REINFORCED    CONCRETE 


103 


b  w2   +  2  AC  m  n   —  2  Ac  m  dc   —    2  At  md 

—  2  At  mn 
bn2    +  2  Ac  mn    +  2  At  mn   =  2  At  md 

+  2  AC  m  dc 

bnz  +2mn(Ac  +  At)  =  2m(Atd+Ac  dc) 
2  mn  (Ac  +  At)        2  m  (At  d  +  Ac  dc) 


,, 

If  we  now  add 


,  T,  «  -A 

to  both  sides 


of  the  equation  it  will  be  possible  to  take  the 
square  root  of  the  left-hand  side  and  further 
simplify  as  follows  : — 

2mn(Ac  +  At)        fm  (Ac  H 
~~b~~  ~  L  b 

2  m  (At  d  +  Ac  dc)     ,    f m  (Ac  +  I 


J 


The  square  root  of  the  left-hand  side  equals 
n  +         -%- —  — ,  therefore  this  will  equal 


/2m(Atd+Acdc)    , 

m  (Ac  +  At) 

2 

,  and 

V                 6 

b         J 

2  m  (Atd  4-  Acdc) 

b 

m  (Ac  +  At 


f 


m 


Having  now  obtained  a  formula  for  the 
position  of  the  neutral  axis,  it  will  be  possible 
to  ascertain  this  and  then  calculate  the 
stress  on  the  concrete.  When  this  has  been 
done  the  stress  on  the  steel  can  be  found  by 
considering  the  elasticity  of  the  two  mate- 
rials and  their  relative  distances  from  the 
neutral  axis.  Thus  t  will  equal  m  times  c 

multiplied  by  -    — ,  or,  expressed  as  a  for- 


d 


and    cs    will    equal 


mula,    t  =  c  m 

n  —  d,, 
cm  —      — . 
n 

It  will  be  advisable  to  give  an  example 
illustrating  the  use  of  these  various  for- 
mulae for  the  purpose  of  checking  ;  the  case 
taken  will  be  that  of  the  beam  previously 
designed  to  carry  a  load  of  10  tons  distri- 
buted in  addition  to  its  own  weight  over 
a  span  of  16  ft.,  when  the  effective  depth 
was  limited  to  20  in.  and  the  width  to  12  in. 
The  tensile  reinforcement  necessary  was 
calculated  to  be  2-269  in.,  and  the  com- 
pressional reinforcement  1-598  in. 

There  will  be  no  need  to  re-calculate  the 
bending  moment,  as  the  size,  and  conse- 
quently the  weight,  of  the  beam  was  avail- 
able at  the  outset.  This  bending  moment 
was  found  to  amount  to  643,200  in.-lb.  The 


first  step  will  be  that  of  finding  the  position 
of  the  neutral  axis. 

2  m  (At  d  +  Ac  de)       fro  (Ac  +A)12 


m(A,;  +At) 


[~m(Ac  +A)"p 
b 


n  = 


X  15  (2'269_  XJ20j+_r6ij8j><_2)_    ,    [15  (1'598 


12 

15  (1-598  +   2-269) 
12 


, 


30  x  48-576 


+  (4-833)2  -  4-833 


n  =   N/121-44  +  23-357  -  4-833 

n  =   x/144W  -  4-833 

n  =  12-03  -  4-833 

n  =  7-197  in. 

Upon  reference  to  the  figures  taken  when 

designing  the  beam,  it  will  be  seen  that  n  = 

7-2  in.,  and  this  will  afford  some  evidence 

to  the  student  that  the  formula  is  satis- 

factory.    The  stress  in  the  concrete  can  now 

be  ascertained  as  follows  :  — 


2BM 


b  n  x  (d  -  o)  +  2  Ac  m  x  ^—       —  ^  x  (d-  dc) 

c  = 

2  x  643200 

(7'197\  /7'197-2\ 

20--g-)+2x  1-598 x  l*x(rriW )x (2° ~2 

1286400 

c  =  12  x  7-197~xl7-6  +2  x  1-598  x  15  x  -71  x  18 
1286400 


c  = 


1520-006  +  612-673 
1286400 


2132-679 
c  =  603  Ib. 

This  is  quite  satisfactory,  as  it  is  only  3  Ib. 
over  the  permissible  limit  of  600  Ib.  per 
square  inch,  and  even  this  would  not  occur 
if  all  the  decimal  figures  were  retained  in 
the  calculations,  but  such  a  method  would 
entail  a  great  deal  of  unnecessary  labour. 
The  stress  in  the  tensional  reinforcement 

d  —  n  , 

=  c  m  -      —  ;  therefore 

20  -  7-197 
t  =  603  x  15  x  --  7497" 

t  =  603  x  15  x  1-77 

t  =  16,000  Ib. 

This  can  be  considered  satisfactory,  and  the 
stress  in  the  compressional  reinforcement 
will  now  be  calculated  by  : 


n  - 


cs=  cm 


104 


REINFORCED    CONCRETE 


cs  =  603  x    15  x 


7-197  -  2 


7-197 

ca  =  603  x  15  x  -72 

c,  =  6,512  Ib. 

The  beam  then,  as  designed,  can  be  con- 
sidered as  satisfactory.  It  will  be  noticed 
that  the  exact  areas  of  the  steel  as  calcu- 
lated were  retained  and  not  substituted  by 
actual  practical  size  bars,  as  these  would  of 
necessity  vary  slightly,  and  this  would  affect 
the  stresses.  The  object  of  this  example  has 
been  to  avoid  all  complications,  as  the 
theory  is  a  little  more  difficult  than  with 
single  reinforced  beams,  and  the  student 
should  become  quite  clear  on  all  the  points 
in  the  formulae  and  then  work  out  various 
examples  and  check  them,  in  some  cases 
working  out  the  bars  required,  and  substi- 
tuting these  for  the  theoretical  areas  before 
finding  the  stresses  in  the  concrete  and  steel. 
All  the  theory  and  formulae  given  will 
apply  equally  to  slabs  and  beams,  bearing 
in  mind  that  the  bending  moments  in  the 
case  of  the  former  must,  if  necessary,  be 
calculated  in  accordance  with  the  rules  pre- 
viously given  if  supported  or  fixed  on  all 
four  edges. 

TEE  BEAMS 

These  beams  are  those  in  which  the  com- 
pressional  resistance  is  partly  or  wholly 
supplied  by  the  slab  above  the  beam,  form- 
ing a  large  T-shaped  section.  Such  a  method 
of  calculation  is  permissible  and  economical 
when  the  slab  and  beam  are  cast  in  one 
operation  with  no  plane  of  cleavage  between 
the  two,  and  reinforcement  is  provided  to 
resist  the  shearing  stress  occurring  along 
the  plane  of  junction. 

When  designing  a  reinforced  concrete 
floor  over  a  very  large  area  it  is  necessary 
to  divide  the  floor  up  into  a  number  of 
panels  by  means  of  beams  which  will  of 
necessity  project  down  below  the  slabs 
which  form  the  panels,  and  sucli  beams  will 
generally  consist  of  main  and  secondary 
beams,  the  former  spanning  the  greater 
distances  and  carrying  the  latter,  which 
intersect  at  right  angles. 

In  smaller  floors  the  design  may  consist  of 
one  set  of  beams  only,  and  in  this  case  the 
slabs  are  considered  to  assist  in  the  resist- 
ance of  such  beams,  while  in  the  case  where 
two  sets  are  employed  it  is  usual  to  con- 
sider the  secondary  beams  only  as  being 
assisted  by  the  slabs,  and  it  is  essential  that 
the  slab  reinforcement,  which  is  at  right 


angles  to  the  beam,  must  extend  through  the 
full  width  of  that  portion  of  the  slab  which 
forms  the  compressional  flange  of  the  beam. 
The  width  of  slab  that  can  be  taken  as 
acting  with  the  beam  will  depend  on  cir- 
cumstances, but  it  must  not  exceed  either 
of  the  following  :  (1)  One-third  of  the  effec- 
tive span  of  the  tee  beam,  (2)  three-fourths 
of  the  distance  from  centre  to  centre  of  the 
ribs  of  the  tee  beams,  (3)  fifteen  times  the 
thickness  of  the  slab,  (4)  six  times  the  width 


•            '                         1 

I 

V 

i 



.  '     ' 

__^4 

} 

-A- 

N5— 

c 

r 

L 

-©-••« 

1  
' 

A 

B 


Fig.   126.— Section  of  Tee  Beam  and  Method 
of  Finding  Total  Compression 

of  the  rib  of  the  tee  beam.  It  will  be  neces- 
sary, therefore,  to  determine  which  is  the 
least  of  these  and  calculate  accordingly. 

The  diagram  presented  by  A,  Fig.  126, 
shows  the  section  of  a  tee  beam,  and  will  be 
helpful  in  explaining  the  value  and  meaning 
of  the  various  symbols  employed. 

There  are  three  cases  that  require  to  be 
considered  according  to  the  position  of  the 
neutral  axis,  and  these  are  indicated  on 
this  diagram.  The  first  is  that  when  the 
neutral  axis  falls  within  the  slab,  the  second 
where  it  coincides  with  the  bottom  of  the  slab, 
and  the  third  where  it  falls  below  the  slab. 


THEORY    OF    REINFORCED    CONCRETE 


105 


The  two  former  cases  offer  no  difficulty, 
as  all  the  formulae  that  have  previously  been 
given  for  beams  with  single  reinforcement 
can  be  employed,  with  the  exception  that  b 
will  now  represent  the  width  of  that  portion 
of  the  slab  acting  as  the  flange  of  the  beam, 
and  in  the  second  case  ds,  which  expresses 
the  thickness  of  the  slab,  will  be  equal  to  n. 

In  the  third  case,  however,  it  will  be  seen 
that  a  portion  of  the  rib  will  be  acting  in 
compression,  and  this  will  affect  the  centre 
of  the  compressional  resistance,  and  conse- 
quently the  lever  arm  of  the  internal  forces. 

In  most  cases  the  amount  of  concrete  in 
the  rib  above  the  neutral  axis  which  assists 
in  compression  is  neglected  in  the  actual 
calculations,  as  the  area  over  which  the 
stress  is  acting  is  comparatively  small,  and 
the  resistance  of  such  a  small  area  situated 
so  closely  to  the  neutral  axis  is  not  such  as 
to  materially  affect  the  economy  of  the 
design.  The  second  report  of  the  R.I.B.A. 
recommends  that  this  method  of  calcula- 
tion should  be  adopted,  and  it  is  therefore 
justifiable  to  consider  the  compression 
strength  in  this  way. 

Upon  reference  to  B,  Fig.  126,  it  will  be 
seen  that  the  total  compression  is  represented 
by  the  shaded  area,  which  has  a  depth  equal 
to  ds  and  width  equal  to  c  at  the  extreme 
top  edge  and  c1  at  the  bottom  edge,  which 
coincides  with  the  bottom  of  the  slab.  The 
area  of  the  beam  over  which  the  stress  is 
considered  as  acting  is  equal  to  b  x  ds, 
where  6  equals  the  width  of  the  flange  and 
d3  the  thickness.  Now.  the  total  compres- 
sion in  the  slab  will  be  the  area  6  x  ds  mul- 
tiplied by  the  stress  per  square  inch.  As 
before  explained,  the  stress  diminishes  uni- 
formly towards  the  neutral  axis,  and  conse- 
quently the  value  of  c1  will  be  less  than  c  in  a 
direct  ratio  with  its  comparative  distance 

from  n.     Therefore  c1  =  c   x    s,  and 

n 

the  mean  stress  acting  over  the  whole  area 

n  —  ds 
c  x  c1         c  +  c  — 


will  equal 


or 


If  the 


area  acting  in  compression  be  multiplied  by 
the  mean  stress  per  square  inch,  then  the 
total  compression  will  be  given.  As  before 
stated,  the  area  =b  x  ds,  therefore  the  total 
compression  = 

,        /          n  -  ds\ 
o  x  ds  x  (c  +  c  • 1 


The  moment  of  the  compression  must  equate 
with  the  bending  moment,  and  the  total 
compression  will  therefore  be  required  to  be 
multiplied  by  the  lever  arm  of  the  internal 
forces.  In  the  case  of  singly  reinforced 

1% 

beams  this  was  always  equal  to  d  —  „,  but 

this  will  no  longer  apply,  as  the  centre  of 
the  compressional  resistance  will  no  longer  be 

situated  at  a  distance  of  ^  from  the  upper 

surface.  The  symbol  a,  however,  is  used  to 
express  the  value  of  the  lever  arm,  and  this 
can  be  employed  to  complete  the  formula 
for  the  moment  of  the  compressional  resist- 
ance, which  will  be  equal  to 

/  n  —  ds\ 

ox   as  x  Ic  +  c  - 

\  f  11' 

s x    a,    and   this 

must  equal  B  M. 

From  this  formula  may  be  deduced  a 
formula  for  finding  the  value  of  c  when  the 
bending  moment  has  been  calculated,  as 
follows  : — 


/  i 
c  6  x  ds  x  1  2 

4^') 

x  a 

"R  M 

2 
c  [6  x  ds  x  (2  n 

-  4)  x 

al         BM 

2BMn 


Therefore  c  =  g  x  rfg  x  (2  n  _  d.)  xa 

The  value  of  the  total  compression  has 
now  been  obtained,  neglecting  the  concrete 
in  the  rib,  and  also  the  method  of  finding 
the  value  of  c  when  the  beam  is  designed. 
It  is  now  necessary  to  show  how  the  lever 
arm  of  the  internal  forces  can  be  ascertainel, 

n 
as  it  will  not  be  d  —  .7,  as  m  previous  cases, 

owing  to  the  total  compression,  as  shown  at 
B,  Fig.  126,  no  longer  being  represented  by  a 
triangle.  As  before,  the  whole  of  the  com- 
pression must  be  considered  as  acting  at 
the  centre  of  gravity,  and  it  is  necessary, 
therefore,  to  find  the  centre  of  gravity  of  the 
shaded  portion,  shown  on  the  diagram. 
The  draft  regulations  of  the  London  County 
Council  state  that  the  lever  arm  can  be 

taken  approximately  as  d  —  ^.     This  is  a 

useful  rule  for  preliminary  calculations  when 
the  steel  is  required  to  be  found  and  the 
position  of  the  neutral  axis  is  not  avail- 
able, but  an  accurate  method  must  be 
deduced  for  exact  calculations.  Now  if 


io6 


REINFORCED    CONCRETE 


the  shaded  portion  was  a  triangle  having  a 
base  equal  to  c  and  a  height  equal  to  ds, 
then  the  centre  of  gravity  would  be  situated 

ds 
at  a  distance  of  -^  from  the  top  edge.    It  is 

necessary  to  consider,  however,  a  figure 
having  four  sides,  where  c1  will  bear  a  cer- 
tain relation  to  c,  and  it  is  obvious  that  the 

d, 

centre  will  be  nearer  to  the  top  than    w> 

and  by  the  principle  of  moments  it  will  be 
found  that  the  actual  position  of  the  point 

ds       2  +  2  c1 
from  the  top  edge  will  equal  ^  x    —  —  —  ^. 

Express  the  distance  from  the  top  edge  by 
the  symbol  a0,  as  shown  in  Fig.  126.  It  has 
previously  been  seen,  however,  that  c1  = 

n  —  dg 

c—      —,  and  by  substitution  the  following 


is  obtained  :  — 


c   +    2  c  x 


n  —  ds 


a,    = 


, 

3  c  n  — -  - 
n 


2cn  — 


c    +    c    x 


n  — 


=    —  x 


™       f 
Therefore  ae    = 


3 n-2ds _  3 dsn  -  2  d,2 
2  w  —  d«      6  n  —  3ds 


j  «.    i 

>  and  the  lever 
' 


6  n  — 
arm  of  the  internal  forces  will  equal  d  —  ac 

3dsn  -2ds* 
- 


The  total  tension  will  equal  t  x  At,  and 
the,  tensional  resistance  will  be  this  value 
multiplied  by  the  lever  arm,  namely  t  x  At 
x  a,  where  a  =  d  —  ac.  This  tensional  resist- 
ance must  also  equal  the  bending  moment, 
and  therefore  t  A  x  d  —  ac  =  B  M,  and  the 
stress  in  the  steel  can  be  found  in  a  beam 
that  has  already  been  designed  by  the  formula 


At  x  d  —  ac' 

Having  ascertained  the  methods  of  deal- 
ing with  both  the  compressional  and  ten- 
sional resistances,  the  next  step  will  be  to 
deduce  some  process  for  finding  the  position 
of  n  in  the  case  of  a  beam  already  designed, 
and  this  is  of  great  importance,  as  there  are 
three  possible  cases,  as  stated  previously. 
In  order  to  be  quite  accurate,  it  will  be 
necessary  to  consider  the  concrete  in  the  rib, 
as  it  is  obvious  that  any  compression  in  this 
portion  would  affect  the  position  of  n,  and 


although  it  is  neglected  in  the  actual  cal- 
culations for  strength,  it  must  be  taken  into 
account  in  the  neutral  axis  formula.  The 
compression  in  the  slab  has  been  shown  to 
be  equal  to 


and  to  this  must  be  added  the  compression 
on  the  leg.    This  will  amount  to 

n  —  ds 
b     ><  C  X   —  — 


where  br  is  equal  to  the  width  of  the  rib, 
giving  a  total  compression  of 

n  —  d.\  in  —  d. 


and  as  the  total  compression  and  tension  are 
equal,  thus 

n  —  d  A  In  —  ds 


This  can  be  simplified  and  deduced  to  give 

a  value  for  n  as  follows  :  — 

2cb  dsn  —  cb  d?  +  c  b,  n  —  cbr  ds 

2~    ^^  t  A.f 
n 

c  (2  b  ds  n  —  b  dsz  +    br  n  —  b,  ds) 

2n 

2  t  At  n 
2bdsn  —  6  ds2  +  b    n  —  b    ds  =  - 


t      m(d 

but  -  =  — 

c  n 

get: 

2  b  ds  n  —  bdsz 


n) 


,  and  by  substitution  we 


z  +  brn  —  brds  = 
2m  At  n  (-I  —  n) 


n 


2  b  ds  n  —  b  ds 

2m  Atnd  —  2m 


bn  —  br  ds  = 


2bds  n  —  bds*  +  br  n  —  brds  =  2  m  At  d 

—  2mAtn 
2  b  ds  n  +  br  n  +  2  m  At  n  =  b  ds2  +  br  d, 

+  2  m  At  d 
n  [2  (b  ds  +  m  A,)  +  br]  =  ds  (b  ds  +  bf) 

+  2  m  At  d 

d,  (b  d,  +  br)  +  2  m  A,  d 

therefore  n  =  —  o~71Tj  —  i  --  /T\  —  i  —  iT~ 
2  (b  ds  +  m  At)  +  br 

Thus  all  the  formulae  necessary  is  obtained 
for  the  checking  of  beams  which  are  already 
designed,  and  it  is  only  necessary  to  ascer- 
tain the  method  to  be  followed  when  design- 
ing the  beams  to  carry  a  stated  load,  and 
this  will  now  be  given. 


THEORY    OF    REINFORCED    CONCRETE 


107 


In  the  designing  of  tee  beams,  there  are 
certain  factors  which  will  be  available  from 
the  previous  calculations,  and  it  will  be 
advisable  to  note  these  in  the  first  instance. 
The  width  of  the  compression  flange,  together 
with  the  thickness,  will  be  given,  as  the  slabs 
will  have  been  designed  previously,  and, 
consequently,  the  values  for  b  and  ds  will 
be  known.  There  remains  for  consideration 
the  method  of  finding  the  thickness  of  the 
rib  ;  the  value  of  d  required  to  give  econo- 
mical design  ;  and  the  amount  of  reinforce- 
ment. The  first-mentioned,  namely,  the 
width  of  the  rib,  will  generally  be  fixed  by 
the  designer  according  to  experience,  as  it 
will  be  influenced  by  the  fact  that  the  rods 
in  the  tensional  area  must  be  properly  spaced 
and  covered ;  and  as  the  value  of  the 
compression  in  the  rib  is  neglected — even 
when  the  neutral  axis  falls  below  the  slab 
— the  calculations  for  the  design  will  not 
be  affected.  There  is  a  minimum  limit,  how- 
ever, and  that  is,  the  width  must  not  be 
less  than  one-sixth  of  the  width  of  the  com- 
pression flange,  but  it  is  seldom  that  this 
minimum  will  be  employed. 

The  method  of  finding  the  economical 
depth  is  one  that  calls  for  a  little  more  con- 
sideration. In  the  first  instance,  it  is  not 
known  whether  the  neutral  axis  will  fall 
below  the  slab  or  not,  and  if  it  should  be 
found  to  do  so  after  the  design  is  complete, 

% 

then  the  lever  arm  will  not  be  d  —  «>  ^ut 

d  —  ac,  and,  unfortunately,  no  definite  rela- 
tion between  ac  and  d  can  be  fixed  unless 
the  amount  of  reinforcement,  and  conse- 
quently the  position  of  the  neutral  axis,  is 
known.  To  keep  matters  as  simple  as  pos- 
sible, however,  it  will  be  advisable  still  to 
consider  n  as  equal  to  -36  d,  as  will  be  the 
case  if  it  falls  within  the  slab,  and  to  con- 
struct a  formula  for  finding  d  accordingly ; 
and  if  the  value  for  d  thus  found  is  not 
quite  accurate,  it  will  form  the  best  guide, 
and  the  necessary  adjustment  can  be  made 
after. 

In  previous  notes  it  has  been  shown  that 

cbn       / ,        n\  . 

BM  =  — o-  x  Id  —  o )  for  a  single  rein- 
forced beam,  and  if  the  neutral  axis  is 
assumed  to  fall  within  the  depth  of  the  slab, 
then  this  formula  will  hold  good.  In  de- 
ducing a  formula  for  giving  the  value  of  d, 
values  may  be  substituted  for  c  and  n,  but 
6  will  be  unknown,  as  it  will  vary  in  practic- 


ally every  example,  and  will  not  necessarily 
bear  any  definite  relation  to  d.  By  putting 
in  the  values  already  known,  it  can  be 
deduced  as  follows  : 

(          ~\ 

-ex      x  n  *  \    -  3) 

c  =  600  and  n  =  -36  d,  then 


2  B  M  =  600  x  b  x  -36  d  x  I d  - 

2  B  M  =  600  x  b  x  -36  d  x  -88  d 
2  B  M  =  180-08  b  d2 
B  M  =  90-04  b  d2 

,72  _      BM          ,  _       /"BIT 

=  90-04  b  ~          :  V  90-04  b 
It  will  be  sufficiently  accurate  if  the  deci- 
mal figures  are  deleted,  giving 


906 

With  regard  to  the  method  of  calculating 
the  amount  of  reinforcement  required,  or 
the  value  of  A/,  this  again  presents  difficul- 
ties, as  it  will  depend  upon  certain  factors, 
one  of  which  will  be  unavailable,  namely,  the 
lever  arm  ;  and  although  it  may  be  taken  as 
being  equal  to  -00675  6  d  when  the  neutral 
axis  falls  within  the  slab,  this  method  will 
not  be  absolutely  accurate  when  the  axis 
falls  below  the  slab.  Another  method  will 
be  that  of  assuming  the  value  of  the  lever 
arm,  and  for  the  preliminary  calculations 

ds 
this  may  be  taken  as  equal  to  d  —  <r  when 

a  safe  approximation  will  result  under 
ordinary  circumstances.  If  this  method  is 
considered  and  accepted,  then  A  can  be 
found  as  follows  : 

B  M  =  At  x  t  x  a,  where  a  =  the  lever 

dst 
arm,  and  this  is  assumed  as  equal  to  d  —  -5 

/        ds\ 

therefore  B  M  =  A«  x  t  x  I  d  —  %  I, 

BM 

then  AC  =  - 


TEE  BEAMS  CONTINUOUS  OR  FIXED  AT 
ENDS 

In  the  case  of  tee  beams  that  are  continu- 
ous or  fixed  at  the  ends,  it  will  be  necessary 
to  calculate  the  strength  at  the  supports  ; 
and  as  the  flange  at  these  points  will  be  in 
the  tensional  area,  its  value  will  be  lost  and 
the  calculations  must  be  made  for  a  rect- 
angular beam  having  a  width  equal  to  the 
width  of  the  rib  only.  If  an  example  is  now 


io8 


REINFORCED    CONCRETE 


taken,  and  the  beam  is  designed  and  checked 
according  to  the  theory  and  formulae  given, 
the  student  will  be  able  to  realise  the  applica- 
tion quite  clearly. 

It  is  required  to  design  a  tee  beam,  fixed 
at  the  ends,  to  carry  a  uniformly  distributed 
load  of  2,000  Ib.  per  ft,  run,  including  the 
weight  of  the  beam  itself,  over  an  effective 
span  of  20  ft.-  The  slab  has  been  designed, 
and  has  a  total  depth  of  6  in.,  while  the 
width  acting  in  compression  with  the  beam 
can  be  taken  at  ten  times  ds,  which  equals 
60  in. 

The  total  load  will  equal  the  load  per  foot 
run  multiplied  by  the  effective  span,  thus 
w  =  2000  x  20  =  40000  Ib.  As  the  beam 
is  fixed  at  the  ends,  the  bending  moment  at 
the  centre  can  be  considered  as  equal  to 

wl  40,000    x    20    x    12 

-jg,    then    B  M   =  —  jg  — 

800000  in.-lb. 

The  first  step  in  the  actual  design  of  the 
beam  will  be  to  calculate  the  depth  by  the 


formula  i. 


B~M 


t 

then  i  - 


=  d  =  x/148  =  say  12  in. 

Let  us  next  find  the  value  of  At  by  con- 
sidering same  as  -00675  6  d.  Then  At  = 
•00675  x  60  x  12  =  A,  =  4-86  in.  If  the 
value  of  the  steel  required  is  also  calculated 
by  the  alternative  approximate  method  of 

considering  the  lever  arm  as  equal  to  d  —  -e 

a 

then  the  use  of  the  formula  will  be  shown. 
This  formula  was  A/  =   — 


Therefore  At  = 


A,  = 


800000 
16000  x  (12  -   f 
800000 


16000  x  9 
At  =    5-55  in. 

The  first  method,  which  gave  4-86  in.,  is 
therefore  the  most  economical,  provided  the 
permissible  stresses  are  not  exceeded,  and 
this  will  be  ascertained  by  checking  the 
beam.  Should  the  steel  provided  by  the  first 
method  be  found  insufficient,  then  it  may 
be  increased  by  any  amount  up  to  that  given 
by  the  second  method,  when  it  will  always 
be  found  to  be  sufficient. 

In  checking  the  beam  the  position  of 
the  neutral  axis  must  first  be  found, 
and  the  formula  for  this  has  been  given, 
namelv : 


(b  d,  +  br)  +  2mAtd 
2(bd,  +  mAt)  +  br      ' 
Assuming  br  to  be  10  in.,  then 

6  (CO  x  6  +  10)  +  2  x  15  x  4-86  x  12 


_ 


2  (60  x  6  +  15  x  4-86)  +  10 
370  +  30  x  58-32 


_ 


2  x  (360  +  72-9)  +  10 

2220  +  1749-6  3969-6 

•=n=-.l..  s-s 


865-8  +  10 
The  thickness  of  the  slab  was  given  as 
6  in.,  and  the  distance  from  the  top  edge 
to  the  neutral  axis  is  4-58  in.,  therefore  the 
latter  falls  within  the  slab,  and  the  beam 
can  be  considered  as  a  singly  reinforced  beam 

Al 

when  the  lever  arm  equals  d  -     Q. 

o 

stresses  can  be  checked  as  follows  : 

2BM 

c  =    -       — ^ —^  ;  therefore 

b  n  : 


The 


2  x  800000 


c  = 


CO   x   4-58  x 

1600000_ 

60  x  4:58  x  1048 
1600000 


(» -  4 


2879-9 
c  =  555  Ib. 

This  is  quite  satisfactory,  and  t  can  now 
be  calculated. 

BM 

t  =   -       — -, —   -^r  ;  therefore 


At  x   ( d  -  ^ 
800000 


t  — 


t  = 


4-86  x  112  -  — 
800000 


4-86  x  10-48 
800000 


50-835 

t  —  15737  Ib.,  which   is  also  quite 
satisfactory. 

The  steel  required  was  4-86  in.,  and  the 
area  of  one  bar  having  a  diameter  of  1|  in. 
=  -994  in.,  and  five  bars  of  this  size  would 
give  an  area  =  -994  x  5  =  4-97  in.,  which 
will  do  very  well.  As  five  rods  are  to  be  used, 
it  will  be  necessary  to  put  them  in  two  rows 
one  over  the  other,  and  the  depth  of  12  in. 
will  be  the  distance  from  the  top  edge  to 
the  centre  between  these  rows.  In  order 
to  get  sufficient  covering  for  the  lower  rods, 
it  will  be  necessary  to  add  3  in.  of  concrete, 
giving  a  total  depth  of  15  in. 


THEORY    OF    REINFORCED    CONCRETE 


109 


In  this  example  the  beam  would  require  to 
be  calculated  at  the  ends,  as  it  was  stated 
to  be  fixed,  and  this  would  be  done  by  using 
the  formulae  given  for  doubly  reinforced 
beams.  As  this  example,  however,  gave 
an  instance  where  the  neutral  axis  fell  within 
the  slab,  it  will  possibly  be  more  useful 
to  give  another  example  where  the  axis  falls 
~3elow  the  slab  and  calculate  the  section  at 
the  centre  and  the  ends  also,  and  the  student 
will  then  have  an  illustration  of  the  procedure 
in  each  case.  It  must  be  borne  in  mind 
that  these  examples  are  taken  at  random, 
and,  as  before  stated,  are  given  for  the  sole 
purpose  of  showing  the  application  of  the 
formulae  and  the  methods  adopted,  and  the 
reader  will  be  well  advised  if  he  studies  the 
working  drawings  of  reinforced  concrete 
work,  and  endeavours  to  follow  the  prac- 
tical application  of  the  theory.  It  is  some- 
what difficult  to  select  an  example  where  the 
ixis  will  fall  below  the  slab  unless  the  load 
to  be  carried  is  made  exceptionally  heavy, 
sr  the  compression  flange  is  taken  as  being 
rery  thin  and  narrow  ;  and  if  the  load  is 
sxcessively  heavy  and  the  beam  is  fixed  at 
the  ends,  then  a  large  amount  of  reinforce- 
ment will  be  required  in  the  compressional 
area  at  the  ends  where  the  flange  is  situated 
in  the  tensional  area,  as  the  steel  will  not 
be  situated  at  any  great  distance  from  the 
neutral  axis  and  cannot  be  stressed  to  its 
full  limit. 

As  an  example,  however,  let  it  be  required 
design  a  tee  beam  securely  fixed  at  the 
mds,  to  carry  a  uniformly  distributed  load 

2,551  Ib.  per  ft.  run,  including  the  weight 
)f  the  beam  itself  over  an  effective  span  of 
14  ft.  The  section  at  the  centre  of  the  span 
is  to  be  calculated  in  the  first  instance,  and 
then  the  section  at  the  ends,  and  the  slab 
in  be  taken  as  4  in.  thick,  and  the  width, 
icting  with  the  beam,  as  ten  times  ds, 

hich  equals  40  in. 

The  total  load  will  equal  the  load  per  ft. 
m  multiplied   by  the  span  =  2551  Ib.    x 
L4  ft.  =  35714  Ib. 

The  bending  moment  at  the  centre  will 
wl  35714  x  14  x  12 

jqual  j---,  therefore  B  M  =  -        — jc> — 

=  499,996  in.-lb.,  say  500,000  in.-lb. 
First  calculate  the  depth,  namely  : 


BM 


therefore  d 


P 

=  V   of 


500000 


90  b  "  'V   90  x  40 

=  -s/139  =  11-8  in.,  say  12  in. 
The  value  of  At  must  next  be  found  by 


taking  same  at  -00375  b  d  ;  therefore  At  = 
•00875  x  40  x  12  =  3-24  in. 

The  section  at  the  centre  is  now  obtained, 
and  before  proceeding  with  the  calculations 
at  the  ends  of  the  span  it  will  be  advisable 
to  find  the  position  of  the  neutral  axis, 
and  see  that  this  falls  below  the  slab,  and 
also  check  the  stresses  on  the  steel  and 
concrete. 

The  position  of  n  will  be  found  by  the 

,  d,  (b  d,  +  br)  +  2  m  At  d 

formula  n  — 0  ,,  j —      — 1-\ — rr — 

2  (b  ds  +  m  At)  +  b,. 

There  is  only  one  unknown  factor  that  must 
be  settled  before  working  out  the  formula, 
and  that  is  the  value  of  &,..  Taking  this,  as 
before,  at  10  in.,  then 

4  (40  x  4  +  10)  +  2  x  15  x  3-24  x  12 


n  — 


n  = 


n  = 


n  = 


2  (40  x  4  +  15  x  3-24)  +  10 
4  x  170  +  30  x  38-88 

2  (160  +  48-6)  +  10 
680  +  1166-4 

427-2 
1846-4 


427-2 
n  =  4-32  in. 

As  the  thickness  of  the  slab  is  only  4  in., 
it  will  be  seen  that  the  axis  falls  -32  in. 
below  same,  and  although  this  is  a  very 
small  distance,  it  will  serve  as  an  example. 

The  position  of  the  neutral  axis  having 
been  obtained,  the  next  thing  will  be  to 
check  the  stress  in  the  concrete,  and  as  the 
axis  falls  below  the  slab  it  will  be  necessary 
to  use  the  formula 


6  x  ds  x  (2  n  —  ds)  x  a 
The  factor  a  is  at  present  unknown,  as  this 
represents   the   lever   arm   of   the   internal 
forces,  and  this  must  be  found  in  the  first 
instance.     It  is  known  that  a  —  d  —  ac  and 

3  d,  n  -  2  42    ..       , 
a,.  =    —  -  S-T—  ,  therefore 


6  n 
3 


x  4  x  4-32  -  2  x  4  x  4 


a,,  = 


6  x  4-32  -  3 
51-84  -  32 


fit-     —  f\r. 


x  4 
19-84 
W92 


25-92  -  12  ~ 
ac  =T42  in. 

As  a  comparison,  it  is  interesting  to  note 
that  if  the  neutral  axis  had  coincided  with 
the  under  side  of  the  slab,  the  centre  of 

compression  would   have  been  equal    to  ^ 

which  would  give  a  distance  of  1-44  in.  from 
the  top  edge  ;  and  as  the  axis  falls  below  the 
slab  by  a  distance  of  -32  in.,  the  centre  of 


no 


REINFORCED    CONCRETE 


compression  is  -02  in.  nearer  the  top  edge, 
and  consequently  the  lever  arm  is  increased 
by  this  amount.  As  already  stated,  a  =d  —  ac, 
therefore  a  =  12  in.  —  1'42  in.  =  10-58  in., 
and  this  value  being  now  available,  c  can  be 
found  as  follows  : 

_  2  x  500000  x  4-32 

~ 


c  = 


40  x  4  x  (2  x  4-32 

4320000 

160  x  4-64  x  10-58 
4320000 


_ 
4)  x  10-58 


c  = 


7854-592 
c  =  549  Ibs. 
This  is  well  below  the  permissible  limit, 


L 

b-J 

n"                           =1 

r 

1         f 

ds-4' 

t 

1 

•1.32 

fcl 

_,     * 

'Z'COVIR.F 

^br 

.      A 
C/-/2" 

.1 

Fig.  127. — Section,  at  Centre  of  Span,  of 
Tee  Beam  Fixed  at  Ends 


and  the  value  of  t  can  be  next  calculated  by 

BM 

the    formula    t  =  -i —  when    a    again 

J\t        X       Ct 

=   d  —   ac,   which   =    10-58   in.,   therefore 

500000  ,  _   500000      _ 

=  3-24  x  10-58  ~  34-2792'  l 
Ib.  This  also  is  well  below  the  permissible 
limit,  and  the  section  at  the  centre  may  be 
considered  as  quite  satisfactory.  If  two  bars 
having  a  diameter  of  \\  in.  are  used,  then 
the  sectional  area  of  steel  will  equal  1-7671 
x  2  =  3-53  sq.  in.,  which  is  more  than 
sufficient. 

The  section  as  designed  is  illustrated  in 
Fig.  127,  and  the  calculations  for  the  sec- 
tion at  the  ends  of  the  span  may  now  be 
considered.  In  this  latter  case,  a  different 
bending  moment  will  exist  having  a  value 

wl  ,       __,      35714x14x12 

equal  to  -TQ,  therefore  B  M  =  -     — j~ — 

=  599995  in.-lb.,  say  600000  in.-lb.  This 
is  a  greater  bending  moment  than  that 
required  to  be  resisted  in  the  centre  of  the 
span,  and  as  the  tension  will  now  be  in  the 
upper  surface,  a  comparatively  large  amount 
of  reinforcement  will  be  required.  The 
section  will  be  calculated  as  a  beam  with 


double  reinforcement  where  the  size  of  the 
concrete  is  fixed.  The  portions  of  the  slab 
on  each  side  of  the  beam  must  be  neglected, 
leaving  a  section  14  in.  deep  and  10  in.  wide. 
Allowing  2  in.  from  the  top  edge  to  the 
centre  of  the  reinforcement,  the  effective 
section  becomes  12  in.  by  10  in.  =  120  sq.  in. 
The  economical  percentage  if  singly  rein- 
forced would  be  -00375  x  120  =  -81  in.  The 
bending  moment  that  such  a  beam  would 

/          n\ 
resist  will  equal  At   x   t  x    Id  —  ^j  and  n 

would  =  -36  d  =  12  x  -36  =  4-32  in.,  then 


B  M  =  -81  x  16000  x  112 3-  j  =  B  M  = 

•81  x  16000  x  10-56  =  136857  in.-lb.  The 
actual  bending  moment  to  be  resisted,  how- 
ever, is  600000,  and  the  excess  which  will 
have  to  be  taken  by  the  compressional  steel 
and  additional  tensional  steel  equals  600000 
- 136857  =  463143  in.-lb.  If  the  com- 
pressional steel  is  placed  2  in.  from  the 
bottom  edge,  then  dc  will  equal  2  in.,  and 
the  lever  arm  or  distance  between  the  two 
sets  of  reinforcement  will  equal  d  —  dc  = 
12  —  2  —  10  in.  The  additional  area  of 
steel  required  in  tension  will  be  found  by  the 
formula  : 

463143 


_  Excess  B  M  _        _ 
At  =  t  x  (d  -  dc)  =  At  ~~ 
463143 


16000  x  (12 -2) 
9  sq.  in. 


'       160000         , 

This  amount,  plus  -81  in.,  the  area  found 
above,  will  give  a  total  amount  of  3-7  in. 
for  the  steel  required  in  the  upper  or  ten- 
sional area.  The  steel  required  in  the  com- 
pressional area  will  be  found  by  the  formula  : 

Excess  B  M 
Ac  ==  cTx~(3  -  dc)' 

The  value  of  cs  must  be  found,  and,  as  before 
stated,  this  will  depend  on  the  position  of 
AC  in  relation  to  the  neutral  axis,  namely, 

t  x  (n  -  dc) 

;  therefore 


cx  = 


d  —  n 
16000  x  (4-32  -  2) 


37120 
~  Cs  =      7-68 


12  -  4-32 

cs  =  4833  Ib. 

It  will  be  seen  that  the  steel  will  be 
stressed  at  a  very  low  figure.  The  formula 
for  the  amount  of  steel  can  now  be  worked 
out  as  follows  : 

463143 

Ac    = 


A,  = 


4833  x  (12  ^ 
463143 


=  Ac  =   9-58  sq.  in. 


THEORY    OF    REINFORCED    CONCRETE 


in 


Before  actually  giving  the  finished  section, 
it  will  be  advisable  to  check  the  stresses,  and 
see  that  they  do  not  exceed  the  permissible 
limits,  and  in  order  to  do  this,  the  position 
of  the  neutral  axis  must  first  be  calculated 
by  the  formula  given  for  double  reinforced 
beams,  namely  : 


/2  m(Atd  +  Ac  dc) 


m  (Ac 


rm(Ac  +  At)J 
b 


— ;  therefore 


/•J  X   15 


X    12  +  i)-58  X   2) 


10 

15  (9\5S  4-  37) 


15   (9'5»  + 

10 


-  v 


30  (44-4  +  19-16)        |"15  x  13-2812 

10  10 

15  x  13-28 


10 


/J905-8        I"199'2"!2  _  199.2 
=  V      10       ~_  LjOj         ~TO~ 
n  =  x/190-68  +  396-7064  -  19-92 
n  =  v/587-38  -  19-92  =  w  =  24-23  -  19-92 
w  =  4-31  in. 

This  compares  very  well  with  the  position 
calculated  according  to  the  economical  per- 
centage when  designing,  namely  4-32  in.  The 
stress  in  the  concrete  can  now  be  found  as 
follows  : 

2BM 


c  = 


bnx 


(n 
d-^ 


c  = 


2   X   600000 


10  X  4-31  X  (l2  -  Ap)  +  2  X  9-58  X  15  X  (4  ^  2)  X  12  -  2 

1200000 


c  = 


c  = 


43-1  x  10-57  +  287-4  x 
1200000 


2-31 


x  10 


1200000 
1995-967 


455-567    +    1540-4 
c  =  601  Ib. 

This  is  quite  satisfactory,   and   t  and  cs 
can  be  proceeded  with. 


-  601  x  15  x  1-784  =  t  =  16082    Ib. 
"i  =;  cm 


cs  =  601  x  15  x 


4-3i 


-  c,  -  4832  Ib. 


All  these  figures  are  quite  satisfactory,  and 
the  beam  can  be  considered  as  being  calcu- 
lated for  the  centre  and  end  sections. 

The  diagram  in  Fig.  128  shows  the  section 


at  the  ends  of  the  span.  As  the  amount  of 
reinforcement  to  take  the  compression  at 
the  ends  was  9-58  in.,  it  will  be  necessary  to 
employ,  say,  six  l|-in.  diameter  rods,  which 
will  give  a  total  area  of  10-5  sq.  in.,  and  these 
must  be  placed  in  two  rows  as  shown.  The 
depth  taken  in  the  calculations  will  be  to 
the  centre  of  the  two  rows,  and  conse- 
quently, it  will  be  necessary  to  allow  3  in. 
of  concrete  below  this  point,  giving  a  total 
depth  of  15  in.  instead  of  14  in.,  which  was 
theoretically  necessary.  This  increase  will 
have  the  effect  of  making  the  resistance 
slightly  in  excess  of  that  calculated,  and 
therefore  it  may  be  adopted.  The  extra  inch 


Fig.   128. — Section,  at  Ends  of  Span,  of  Tee 
Beam  Fixed  at  Ends 

of  covering  is  not  indicated  on  the  section 
illustrated  in  Fig.  127,  as  it  is  not  necessary, 
and  the  under  side  of  the  beam  could  be 
formed  with  a  very  slight  camber,  which 
would  improve  rather  than  detract  from  the 
appearance  of  the  beam.  The  question  of 
bending  up  certain  bars  from  the  lower  sur- 
face to  the  upper  near  the  supports,  and  the 
calculations  for  shearing  stress,  are  not  con- 
sidered here. 

TEE  BEAMS  WITH  DOUBLE  REINFORCE- 
MENT 

With  regard  to  tee  beams  with  double 
reinforcement,  it  is  not  considered  necessary 
to  give  any  particular  examples  for  this  class 
of  calculation,  as  the  same  formulae  and 
principles  will  apply  as  were  given  for  beams 
with  double  reinforcement  when  the  neutral 
axis  falls  within  the  slab  or  coincides  with 
the  under  side,  and,  as  previously  stated, 
these  are  the  most  common  cases.  It  must 
be  borne  in  mind,  however,  that  b  will  repre- 
sent the  width  of  the  flange  acting  with  the 
beam  in  compression.  When  the  axis  falls 
below  the  slab,  the  beam  can  be  designed 
upon  the  assumption  that  the  concrete  in 
the  rib  is  to  be  neglected,  and  the  various 


112 


REINFORCED    CONCRETE 


formulae  previously  given  can  be  used,  and 
if  necessary  slight  adjustments  can  be  made 
after  the  preliminary  calculations  have  been 
made.  Where  double  reinforcement  is  em- 
ployed, the  two  sets  should  always  be 
connected  by  binding. 

SHEARING    STRESS   AND   ADHESION 

The  question  of  the  shearing  stress  and 
adhesion  has  been  omitted  up  to  the  present 
in  order  to  simplify  matters  and  enable  the 
student  to  become  quite  familiar  with  the 
principles  of  designing  for  tensional  and 
compressional  stresses  ;  but  the  provision 
of  steel  in  practically  all  concrete  beams  to 
resist  shear  is  absolutely  necessary,  and  this 
question  has,  in  the  past,  received  far  too 
little  attention.  When  the  use  of  reinforced 
concrete  was  more  or  less  in  its  infancy, 
this  side  of  the  design  was  so  little  dealt 
with  that  in  nearly  all  cases  of  tests  applied 
to  beams  where  failure  occurred,  such  failure 
was  due  to  insufficient  provision  against  shear 
or  diagonal  tension,  and  it  was  this  fact  that 
caused  the  more  careful  designers  to  take  up 
this  part  of  the  subject  more  seriously. 

The  draft  regulations  of  the  London 
County  Council  make  provision  for  shear 
members  as  follows  :  "  All  beams  shall  be 
provided  with  adequate  shear  members, 
and  such  shear  members  shall — (a)  Be 
spaced  according  to  the  distribution  and 
intensity  of  the  shearing  stresses  ;  but  the 
distance  from  centre  to  centre  of  the  shear 
reinforcement  at  any  part  of  the  beam  shall 
not  exceed  the  effective  depth  of  the  beam. 
(6)  At  least  extend  from  the  centre  of  the 
tensile  reinforcement  to  the  centre  of  pres- 
sure in  the  concrete  under  compression. 

(c)  Be  passed  under  or  round  the  tensile  rein- 
forcement, or  be  otherwise  secured  thereto. 

(d)  Have  a  mechanical  anchorage  at  both 
ends,  or  they  shall  have  a  mechanical  bond 
with  the  concrete  throughout  their  length." 

The  first  point  for  consideration  will  be 
that  dealing  with  the  intensity  and  distri- 
bution of  the  shearing  stresses.  The  shear- 
ing tendency  is  due  to  the  opposition  of  the 
weight  and  the  reactions,  and  as  the  great- 
est opposition  will  occur  at  the  supports 
where  the  total  weight  meets,  as  it  were,  the 
total  reaction,  it  is  at  this  point  that  the 
greatest  shearing  stress  is  found,  and  the 
stress  at  any  intermediate  point  will  equal 
the  reaction  at  the  support,  minus  any  por- 
tion of  the  load  situated  between  this  point 
and  the  support  under  consideration.  Thus 


the  amount  of  shearing  stress  is  affected  by 
the  amount  and  nature  of  the  load,  but  not 
by  the  span  ;  and,  cons  quently,  it  is  in 
the  case  of  beams  which  carry  heavy  loads 
over  a  short  span  that  the  shear  becomes 
the  most  important  consideration.  The 
shearing  stress  acts  both  horizontally  and 
vertically,  and  at  any  point  in  the  beam 
these  two  actions  will  be  equal,  and  as  a 
result  the  shear  members  are  often  placed 
at  an  angle  of  45  degrees  to  resist  the 
resultant  of  the  two  forces.  The  distribu- 
tion of  the  stresses  over  the  area  of  the 
beam  is  somewhat  curious,  as  the  combined 
action  of  the  bending  moment  and  shearing 
force  causes  the  lines  of  maximum  stress  to 
assume  a  curved  form  ;  and  the  shearing 
stress  will  be  greatest  where  the  tension 
and  compression  are  nil,  namely  at  the 
neutral  axis. 

Diagonal  tension  is  often  expressed  as 
shear,  whereas  it  is  dependent  on  the  shear 
and  the  longitudinal  tension  for  its  inten- 
sity. The  actual  moment  of  this  intensity 
cannot  be  ascertained  with  any  certainty, 
and,  consequently,  it  is  usual  in  practice  to 
calculate  the  vertical  shear,  and  take  this 
as  the  measure  of  the  diagonal  tension,  or, 
at  least,  provide  sufficient  steel  for  this  pur- 
pose, when  the  beam  should  be  efficiently 
designed.  The  shearing  stress,  as  previously 
stated,  is  not  uniformly  distributed  over  the 
area  of  the  section,  but  is  greatest  where  the 
longitudinal  stresses  are  least,  namely,  at 
the  neutral  axis,  and  in  an  ordinary  homo- 
geneous beam  the  stress  diminishes  from 
the  axis  to  the  outer  edges,  where  it  becomes 
nil,  assuming  a  parabolic  curve,  as  shown  in 
diagram  A  (Fig.  129).  If  S  equals  the  total 
shear  at  a  vertical  section,  then  the  maxi- 
mum shear  which  occurs  at  N  A,  and  is  ex- 
pressed by  m  s,  can  be  found  as  follows  : — 
The  area  of  the  shaded  portion  represents 
the  total  shear  on  the  section  =  S,  and  the 
area  of  this  figure  =  §  m  s  x  d,  therefore 

S  3  S 

|  m  s  x  d  =  S  and  §  m  s  =  -7,  or  m  s  =  ~-j. 

If  a  diagram  is  set  up  as  given  at  A  (Fig. 
129),  then  the  actual  stress  at  any  point  in 
the  section  can  be  determined. 

It  is  unimportant  in  most  cases  to  know 
the  exact  distribution  of  the  stress,  but 
this  instance  is  given  rather  to  make  the 
theory  more  explicit. 

Now,  in  the  case  of  a  reinforced  concrete 
beam,  the  distribution  of  the  shearing  stress 
is  somewhat  different,  and  the  reason  for 


THEORY    OF    REINFORCED    CONCRETE 


this  is  the  fact  that  the  concrete  above  the 
axis  only  is  called  upon  to  resist  longitudinal 
stress,  that  below  the  axis  being  neglected, 
and  all  the  tension  is  taken  as  coming  upon 
the  steel.  The  result  of  this  assumption  is 
that,  although  the  shear  above  the  neutral 
axis  will  gradually  diminish  towards  the 
outer  edge,  below  this  point  it  will  be  uni- 
formly distributed  over  the  section  as  shown 
in  diagram  B  (Fig.  129).  As  the  upper  part 
of  this  figure  gives  a  parabolic  curve,  the 
equivalent  area  over  which  the  stress  acts 
above  the  neutral  axis  will  equal  two- 
thirds  bn,  and  below  the  axis  6  x  (d  -  n), 


where  b  =  the  width  of  the  section,  and  s  = 
the  maximum  shear  per  square  inch.  In 
dealing  with  the  calculations  for  shearing 
stress  it  is  also  necessary  to  consider  that 
of  the  adhesion  between  the  steel  and  the 
concrete,  as  the  two  materials  can  only  act 
together  when  this  adhesion  is  not  overcome. 
The  permissible  allowance  for  concrete  in 
shear  has  been  given  in  the  table  of  working 
stresses  as  60  Ib.  per  square  inch,  while  the 
allowance  for  adhesion  between  steel  and 
concrete  is  given  as  100  Ib.  per  square  inch, 
and  the  shearing  stress  across  the  section 
divided  by  the  total  circumference  of  the 
rods  or  shear  members  must  not  exceed  this 
amount.  If  the  perimeter  or  circumference 
of  the  bars  be  expressed  by  0,  then  the 
intensity  of  stress  per  square  inch  around  the 

S 
bars  will  be  equal  to  — -, —  -rr~.      In  the 


--A 


o 


('-9' 


Fig.   129. — Determining  Stress  in  Ordinary 
and  in  Reinforced  Concrete  Beams 

therefore  the  total  equivalent  area,  taking 
the  shear,  will  equal  6  x  (  d  -  ^}.    Where  S 

equals  the  total  shearing  stress  acting  over 
the  section,  the  greatest  stress  per  square 

o 

inch  will  be  equal  to 


b    X 


and  this 


is  expressed  by  the  symbol  s. 

In  designing  the  shear  members,  it  is  neces- 
sary to  consider  the  amount  of  stress  that  is 
acting  over  a  portion  of  the  section  equal  to 
1  in.  in  depth,  and  with  a  width  equal  to  6, 

S 


and  this  value  will  equal 


or  &  x  s, 


designing  of  shear  members,  it  is  usual 
and  advisable  to  utilise  stirrups  or  rods  of 
one  section  in  the  same  beam  and  vary  the 
spacing  according  to  the  amount  of  stress 
to  be  resisted.  As  the  greatest  shear  will 
always  occur  at  the  abutment,  the  stirrups 
will  be  fairly  close  together  here,  and  will 
gradually  increase  in  spacing  towards  the 
A  point  at  which  they  are  not  required.  To 
resist  the  vertical  shear,  there  are  the  con- 
crete and  the  horizontal  reinforcement, 
but  in  the  case  of  the  horizontal  shear  it 
will  in  most  cases  be  necessary  to  provide 
vertical  members,  as  mentioned  above,  or 
crank  up  some  of  the  tension  rods  at  a  suit- 
able point,  or  provide  both,  and  it  will  be 
necessary  presently  to  illustrate  the  method 
of  determining  the  positions  of  the  shear 
members  to  suit  the  varying  shearing  stress. 

To  illustrate  the  use  of  the  formulae  already 
given,  examples  may  be  taken  and  their 
application  shown.  In  the  case  of  ordin- 
ary slabs  and  rectangular  beams  with  single 
reinforcement  which  are  accurately  calcu- 
lated to  resist  the  longitudinal  stresses,  it 
will  be  found  that  sufficient  concrete  is  pro- 
vided in  the  section  to  withstand  the  shear- 
ing stress,  and,  consequently,  no  special 
shear  members  are  theoretically  necessary, 
but  for  practical  reasons  they  should  be  pro- 
vided, as  they  assist  in  holding  the  con- 
'  crete  together,  and,  furthermore,  they  will 
be  required  under  the  regulations  of  the 
authority  under  whose  jurisdiction  the 
building  will  come. 

As  an  illustration,  consider  a  beam  sup- 


REINFORCED    CONCRETE 


ported  at  the  ends,  which  is  required  to  carry 
a  uniformly  distributed  load  of  3,000  Ib.  per 
foot  run,  including  the  weight  of  the  beam 
itself,  over  an  effective  span  of  16  ft.  t  The 
total  load  will  equal  the  load  per  foot  run, 
multiplied  by  the  span  =  3,000  Ib.  x  16  ft. 

w  I 
=  48000  Ib.    The  bending  moment  =  -g-  = 


48000  x  16  x  12 


=  1,152,000  in.-lb.     Gal- 
<j 

culating  the   economical  size  of  d  by  for- 

,    3  /EM.       ,      3  /1 152000 
mula,  we  have  d  =\/  -^=-  =  d  —^/  — — — 

'  0  I  0  I 

=  d  =  V7  20210  =  say  28  in.  The  breadth 
should  equal  -6  d,  therefore  b  =  -6  x  28  in. 

=  16-8  in.,  say  17  in.  A,  =  -00675,  b  d  = 
•00675  x  17  x  28  =  3-213  sq.  in.  The 
greatest  shear  stress,  as  previously  stated, 
will  occur  at  the  supports,  where  it  will  be 
equal  to  the  reaction.  This  value  will  be 

W        48000 
-  =  —5—   =  24000  Ib.     The  load  being 

a  a 

uniformly  distributed,  the  shear  will  diminish 
from  the  support  to  the  centre  of  the  span, 
where  it  will  be  nil,  and,  consequently,  if 
calculations  are  made  for  the  position  where 
the  greatest  shear  occurs,  and  the  beam  is 
found  to  be  sufficient,  it  will  be  obvious  that 
it  will  be  safe  at  all  points.  The  greatest 
shear  is  represented  by  S,  and  this  equals 
24,000  Ib.,  while  the  formula  for  finding  the 
maximum  stress  per  square  inch  has  been 
a 

,  where  s  =  the 


gven  as  s  = 


b  x 


maximum  stress  per  square  inch,  which  must 
not  exceed  60  Ib.,  or  shear  members  will  be 
theoretically  required.  Working  this  out,  we 

24000  24000 

get  s  =  - 


17  x 


24000 


10-08 


\ 


17  x  24-64 


=  57-5  Ib.,  which  is  below  the  per- 

o 

missible  limit.  The  value  of  n  was  taken  at 
•36  d,  as  the  economical  percentage  of  rein- 
forcement was  used,  therefore  n  =  -36  x  28 
=  10-08  in.  as  given. 

The  adhesion  or  shearing  stress  intensity 
round  the  reinforcing  bars  will  now  be  con- 
sidered ;  it  must  not  exceed  100  Ib.  per  square 
inch.  The  amount  of  steel  required  was  found 
to  be  3-213  sq.  in.,  and  if  bars  having  a 
diameter  of  1 J  in.  are  used,  the  area  of  which 
equals  -994  sq.  in.,  four  of  these  would  give 
an  area  of  -994  x  4  =  3-976  sq.  in.,  which 


is  more  than"  sufficient.     To  test  the  adhe- 

S 
sion,  the  formula -, —     -^Y  must  be  used. 


0 


-  9 


The  circumference  of  one  IJ-in.  bar  =  3*5343 
sq.  in.,  and  the  total  circumference  of  the 
four  bars  =  3-5343  x  4  -  14-1372  sq.  in. 
Then  the  intensity  per  square  inch  = 
24000  24000 

W08\  "14-1372  x  24-64 
14-1372   x 

24000 


—  68-8  Ib.,  which  is  well  below 


348-3406 
the  limit. 

This  example  should  be  sufficient  to  show 
the  method  of  procedure  for  a  simple  case, 
and  indicate  the  truth  of  the  statement  that 
theoretically  no  special  shear  members  are 
necessary  with  a  single  reinforced  beam 
designed  with  economical  proportions.  As 
the  draft  regulations  of  the  London  County 
Council  state  that  shear  members  must  be 
provided,  and  the  distance  apart,  from  centre 
to  centre,  must  not  exceed  the  effective 
depth  of  the  beam,  we  could  adopt  stirrups 

1  in.  wide  and  T*F  in.  thick,  and  place  same 
apart  at  some  distance,  not  exceeding  28  in. 
The  span  being  16  ft.,  this  can  be  divided  up 
into    the   requisite   number    of   spaces,    as 

192 
follows  :    16   x  12  =  192  in.,  therefore  -^ 

=  say,  seven  spaces.  If  the  stirrups  are 
arranged  to  give  a  space  at  the  centre  of  the 
span,  six  stirrups  only  will  be  required.  The 
next  example  will  be  one  in  which  the  con- 
crete is  insufficient  alone  to  resist  the  shear, 
and  special  shear  members  are  therefore 
theoretically  necessary. 

As  a  further  example,  let  it  be  required  to 
design  a  beam  to  carry  a  uniformly  dis- 
tributed load  of  2  tons  per  foot  run  over  an 
effective  span  of  16  ft.,  such  load  to  be 
inclusive  of  the  weight  of  the  beam  itself, 
which  is  limited  in  size  to  20  in.  effective 
depth  and  12  in.  wide.  Total  load  = 

w  I  . 

2  x  2240  x  16  =  71680  Ib.     B  M  =  -g-  if 

beam  is  supported  only,  therefore  B  M  = 
71680  x  16  x  12 

8 

size  of  the  beam  =  20  in.  x  12  in.,  and  the 
economical  percentage,  if  singly  reinforced, 
would  be  20  x  12  x  -00675  =  1-62  sq.  in. 
Next  find  the  moment  of  resistance  of  such 
a  beam,  and  if  necessary  provide  additional 


THEORY    OF    REINFORCED    CONCRETE 


reinforcement  to  take  the   excess   bending 
moment,  as  explained  in  doubly  reinforced 

beams.     M  R  =  B  M  =  At   x  t  x  ( d  -  ^ ), 

and  n  =  -36   x  20  =  7-2  in.,  therefore  B  M 

=  1-62    x    16000  x  (20  -   -^-]  -  456192 

in.-lb. 

The  actual  B  M  =  1720320  in.-lb., 
therefore  excess  B  M  -  1720320  -  456192 
in.-lb.  =  1264128  in.-lb.,  and  this  must 
be  provided  for  by  compressional  and 
additional  tensional  reinforcement.  Then 

x        Excess  B  M  _1264128 

•"•«    —  4~~~"/,j~~~.i\  ~  •".(   =  -•  CAAn  ~ 


A,1     - 


1264128 


-    4-38 


1 6000  x  (20-2) 
sq.    in.       This 


288000 

amount  must  be  added  to  the  value  given 
above  of  1-62  sq.  in.,  making  a  total  amount 
for  the  tensional  reinforcement  of  6'00 
sq.  in.  The  compressive  reinforcement  will 
be  found  by  the  formula  : 

Excess  B  M  t  x  (n  -  dc) 

-     — ,  where  c  = 


x    (d  -  dey 


,           16000  x  (7-2  -  2) 
equals  c,  =          ^       „„ '  =  c,  = 


d  -  n 
83200 


Ac    = 


20  -  7-2 
=  6500  Ib.     Therefore, 
1264128 


=  Ac  = 


1264128 
117000 


6500  x  (20  -  2) 
AC  =  10-8  sq.  in. 

There  is  no  need  to  explain  the  method  of 
checking  the  longitudinal  stresses,  as  that 
has  already  been  done  in  previous  examples, 
but  the  provision  for  shear  may  now  be 
investigated.  The  total  load  is  equal  to 
71,680  Ib.,  and  the  greatest  shearing  stress 
will  occur  at  the  supports,  where  it  will 

,  W        71680 

equal  -y  =  — ^—  =  35840  Ib.  The  allow- 
ance for  shear  on  the  concrete  must  not  ex- 
ceed 60  Ib.  per  square  inch,  and  therefore  the 
value  of  the  section  to  resist  the  shear  will 
be  equal  to  the  equivalent  shear  area,  which 

(n\ 
d   "  ~o)    x  60  Ib.,  therefore  resistance 
/ 

of  concrete   =  12 


x  (20   -    7J 


x    60   = 

12  x  17-6  x  60  =  12672  Ib.  This  value 
being  less  than  the  actual  shear  coming  upon 
the  section,  it  is  obvious  that  there  will  be 
an  excess  equal  to  35,840  --  12,672  Ib.  = 
23,168  Ib.,  which  must  be  met  by  steel  shear 
members.  The  working  stress  for  steel  in 
shear  is  given  as  12,000  Ib.  per  square  inch, 


and  the  area  of  steel  required  will  equal 
23168 

V2000  ~  ^  ^  s^'  m*  ^  ^e  beam  is  SUP" 
ported  at  the  ends  only,  and  the  tensional 
reinforcement  at  the  centre  of  the  span 
equals  6-00  sq.  in.,  it  will  be  clear  that  as 
the  bending  moment  diminishes  towards  the 
supports  the  tensional  reinforcement  may  be 
reduced,  and  consequently  one  or  more  of 
the  bars  may  be  cranked  at  a  point  where 
the  shear  on  the  concrete  becomes  over  the 
permissible  limit  and  carried  through  the 
shear  area  in  such  manner  that  it  takes  up 


Fig.  130. — Vertical  Shear  in  Reinforced 
Concrete  Beam  carrying  Uniformly 
Distributed  Load 

the  excess  shear.  Reference  to  Fig.  130  will 
show  how  the  shearing  stress  decreases  from 
the  maximum  value  of  35,840  Ib.  at  the  sup- 
port to  nil  at  the  centre,  the  diagram  illus- 
trating one  half  of  the  beam  only,  and 
consequently  the  total  shear  on  one  half  the 
beam  is  represented  by  the  triangle  ABC. 
The  shearing  value  of  the  concrete  in  section 
is  12,672  Ib.,  and  if  this  amount  is  set  down 
to  scale  from  B,  as  indicated,  and  a  hori- 
zontal line  is  drawn  through  to  the  line  A  c, 
then  the  position  where  it  cuts  this  line,  as 
shown  by  the  dotted  vertical  line,  will  be 
the  point  at  which  the  shearing  stress 
becomes  greater  than  the  concrete  alone  is 
capable  of  taking. 

This  point  may  be  found  by  calculation 
as  follows  :   The  shear  at  any  point  is  equal 


n6 


REINFORCED    CONCRETE 


to  the  load  at  the  support  minus  the  load 
situated  between  that  point  and  the  sup- 
port. Now  the  load  at  the  support  =  35,840 
lb.,  and  as  the  value  of  the  sectional  concrete 
to  take  the  shear  is  only  12,672  lb.,  it  is 
necessary  to  move  outwards  from  the  sup- 
port along  the  beam  until  sufficient  load  has 
been  placed  between  us  and  the  support  to 
reduce  the  load  to  this  amount.  It  will  be 
necessary,  therefore,  to  move  outwards 
until  an  amount  of  35,840  12,672  = 
23,168  lb.  has  been  passed  over.  The  load 
is  equal  to  2  tons,  or  4,480  lb.  per  ft.  run,  and 
the  distance  from  the  support,  therefore, 

OQ1  £Q 

equals    TTOQ     =  5  ft.  2  in.    If  the  diagram 


be  set  out  to  scale,  as  in  Fig.  130,  this  can  be 
checked.  It  is  from  this  point  that  the 
necessary  tensional  bars  can  be  cranked  up, 
and  from  this  point  to  the  support  provision 
for  horizontal  shear  must  be  made. 

On  considering  a  horizontal  plane  taken 
along  the  beam  at  any  level  below  the 
neutral  axis,  and  finding  the  total  stress 
intensity  existing  along  this  plane,  sufficient 
stirrups  can  be  calculated  to  take  the  total 
excess  of  shear,  and  by  keeping  all  the 
stirrups  of  one  size,  they  can  be  spaced  at 
varying  distances  apart  and  be  thus  equally 
stressed.  Taking,  firstly,  the  total  shear  at 
the  supports,  which  equals  35,840  lb.,  it  is 
required  to  find  the  shear  on  a  portion  1  in. 
deep  and  with  a  width  equal  to  b  by 

S  35840          35840 


d 


20  - 


7-2 


17-6 


Of  this  amount  the  concrete  is  capable  of 
taking  an  amount  equal  to  the  width  of  the 
section  multiplied  by  60  lb.,  therefore  value 
of  concrete  =  12  x  60  =  720  lb.,  leaving  an 
excess  of  2,036-36  -  720  =  1,316-36  lb.  Now 
this  excess,  which  has  to  be  taken  by  the 
steel,  will  gradually  diminish  from  the  sup- 
port towards  the  centre  of  the  beam  until  it 
reaches  a  point  5  ft.  2  in.  from  the  support, 
when  it  will  be  nil,  as  the  concrete  alone  is 
capable  of  taking  all  the  shear  from  this 
point  to  the  centre  of  the  beam.  This  is 
clearly  shown  in  the  diagram  in  Fig.  131. 
Having  now  obtained  the  maximum  excess 
which  requires  to  be  taken  by  the  steel, 
and  the  length  of  beam  over  which  this  is 
acting,  the  total  excess  may  be  determined 
by  taking  the  mean  of  the  excess  at  the  sup- 
port and  at  a  point  5  ft.  2  in.  from  the  sup- 
port, and  multiplying  same  by  the  length 


over  which  it  is  acting  This  amount  will 
obviously  be  equal  to  the  area  of  the  triangle 
shown  as  DBF  in  Fig.  131,  which  equals 

DE;DF   _    1316'3!!  X  6g  =   40807-16 

Zi  A 

in.-lb.  This  is  the  total  horizontal  shear, 
therefore,  for  which  steel  must  be  provided, 
and  as  the  allowance  for  steel  in  shear  is  given 
as  12,000  lb.  per  square  inch,  the  sectional 

.     ,      ...          .,40807 
area  required  will  equal?  1  0Ann  =  3-4  sq.  in. 


If  it  is  decided  to  use  stirrups  1  in.  wide'and 
J  in.  thick  the  sectional  area  would  equal 
•25  sq.  in.,  and  as  each  stirrup  will  have  two 


Fig.  131. — Horizontal  Shear  in  Reinforced 
Concrete  Beam  carrying  Uniformly 
Distributed  Load 


wings,  the  area  provided  by  each  stirrup  will 
equal  -5  sq.  in. 
The   total   number   of   stirrups   required 

will    therefore    equal     -^-    =    say    7,    and 

these  must  be  spaced  to  give  a  uniform  stress 
on  each  one.  This  may  be  done  graphically, 
as  illustrated  in  Fig.  131,  where  a  semicircle 
is  set  up  over  the  length  of  beam  in  which 
the  stirrups  are  to  be  placed,  and  the  length 
is  also  divided  up  into  fourteen  equal  divi- 
sions, being  twice  the  number  of  stirrups 
to  be  employed.  Perpendicular  lines  are 
then  erected  to  cut  the  semicircle  as  shown,, 
beginning  at  one  end  and  setting  up  a 
line  from  the  first  division  mark,  and 
then  from  every  alternate  mark  until  the 
opposite  end  is  reached,  and  the  last  line 
erected  over  the  point  marking  the  end  divi- 


THEORY    OF    REINFORCED    CONCRETE 


117 


sion.  The  points  at  which  these  perpendicu- 
lar lines  cut  the  semicircle  are  then  trans- 
ferred to  the  horizontal  line,  marking  the 
top  of  the  beam  with  the  varying  radii  as 
shown,  when  the  positions  of  the  stirrups 
will  be  obtained.  The  object  of  thus  spacing 
the  stirrups  is,  of  course,  to  divide  up  the 
shearing  stress  represented  by  the  triangle 
DBF  into  equal  portions,  and  if  the  lines 
representing  the  stirrups  are  continued  down 
to  cut  the  triangle  as  indicated,  it  will  be 
seen  that  it  is  divided  up  into  divisions  of 
varying  width  which  have  more  or  less 
equal  areas. 

To  comply  with  the  draft  regulations  of 
the  London  County  Council  it  would  be 
necessary  to  introduce  the  additional  stirrups 
shown  by  dotted  lines.  These  stirrups  are 
spaced  20  in.  apart,  equal  to  the  effective 
depth  of  the  beam,  and  should  have  the 
ends  turned  out  to  afford  a  good  bond  with 
the  concrete.  In  the  case  of  inclined  stirrups 
the  best  angle  for  these  will  be  45°,  and  it 
is  necessary  that  they  should  be  firmly 
attached  to  the  horizontal  rods  if  they  do 
not  actually  form  a  part  of  same.  The 
spacing  for  these  can  be  found  in  a  similar 
manner  to  that  employed  for  vertical 
stirrups,  except  that  the  inclined  line  must 
be  drawn  through  the  point  at  which  the 
vertical  division  cuts  the  neutral  axis. 
There  is  no  doubt  that  the  best  method  is 
that  where  vertical  stirrups  are  employed 
together  'with  a  certain  number  of  bent -up 
bars,  the  method  being  effective  in  the 
resistance  of  diagonal  tension. 

COLUMNS 

The  theory  and  calculations  in  the  case 
of  columns  must  be  divided  into  two  dis- 
tinct sections,  namely,  short  columns  and 
long  columns,  the  reasons  for  dealing  with 
compression  members  in  this  manner  having 
previously  been  given  in  the  notes  on  columns 
in  the  preceding  chapter.  In  the  case  of 
reinforced  concrete  work  the  column  may 
be  considered  as  belonging  to  the  first  sec- 
tion where  the  length  does  not  exceed 
eighteen  times  the  least  diameter,  and  as 
such  short  columns  are  considered  to  fail 
by  direct  crushing  only,  the  full  working 
stress  may  be  adopted,  and  the  design  is 
comparatively  simple.  These  will  be  con- 
sidered in  the  first  instance  after  a  few 
general  remarks  relating  to  reinforced  con- 
crete as  a  material  for  columns. 

The  steel  is  usually  placed  in  the  form  of 


vertical  bars  spaced  equidistant  on  the  cir- 
cumference in  the  case  of  a  circular  column, 
and  at  each  corner  in  the  case  of  a  rect- 
angular section,  except  in  the  case  of  rect- 
angular sections  where  the  sides  are  un- 
equal as  given  below  ;  and  these  vertical 
bars  must  be  linked  or  bound  together  with 
some  form  of  lateral  reinforcement  to  pre- 
vent the  possibility  of  the  load  causing  the 
bars  to  burst  outwards  through  the  com- 
paratively thin  covering  of  concrete  coming 
over  them.  The  steel  rods  themselves  assist 
the  concrete  core  when  under  the  load,  and 
prevent  the  bursting  tendency,  and  to  do 
this  effectively  they  must  be  well  connected, 
and  by  links  which  are  close  together,  thus 
forming  practically  a  steel  cage  which  confines 


A     -i- 


1  1 

I  I 

p 

•  -1 

',  ,-L-4    jj 

h 
'l 

Fig.   132. — Diagrams  Showing  need  for  Lateral 
Reinforcement  in  Columns 

the  concrete  and  allows  it  to  develop  its  full 
strength.  It  will  be  readily  understood  that 
in  the  case  of  a  concrete  column  with  ver- 
tical rods  only  there  will  be  a  great  ten- 
dency for  the  rods  to  act  as  shown  in  an 
exaggerated  manner  by  dotted  lines  in  dia- 
gram A  (Fig.  132),  whereas,  supposing  two 
intermediate  links  were  introduced,  this 
tendency  would  be  lessened,  and  the  rods 
could  only  bend  as  shown  in  diagram  B. 
If  sufficient  intermediate  links  are  intro- 
duced the  tendency  will  be  overcome  alto- 
gether, and  it  will  be  obvious,  therefore, 
that  the  lateral  reinforcement  is  an  impor- 
tant factor  in  the  design. 

The  draft  regulations  of  the  iondon 
County  Council  state  that  the  total  cross 
sectional  area  of  the  vertical  reinforcement 
shall  not  be  less  than  0-8  per  cent,  of  the 
nrea  of  the  hooped  core,  and  the  volume  of 
lateral  reinforcement  shall  not  be  less  than 
0-5  per  cent,  of  the  volume  of  the  hooped 


n8 


REINFORCED     CONCRETE 


core.  The  effective  diameter  must  be  mea- 
sured from  the  outside  of  the  outermost  ver- 
tical reinforcement,  and  it  must  be  measured 
in  the  direction  of  the  lateral  supports 
which  determine  the  length  of  the  pillar. 

When  the  column  is  such  that  the  laterals 
are  rectilinear,  there  must  be  at  least  four 
lines  of  vertical  reinforcement  throughout 
the  entire  length,  and  if  the  laterals  are 
curvilinear,  there  must  be  at  least  six  lines 
of  vertical  reinforcement,  and  the  diameter 
of  the  rods  in  any  case  must  not  be  less  than 
£  in.,  and  the  pitch  of  the  laterals  must  not 
exceed  six-tenths  of  the  effective  diameter 
of  the  pier.  In  the  case  where  rectangular 
piers  are  used  in  which  the  ratio  between 
the  greater  and  lesser  diameter  exceeds 

EEC  Jl  LINEAR  LAjER 


C-f 

A 

(^ 

«& 

\'\ 

~  "JTl 

^ 

t 

]HI5  ODTANCf 
EXCEED  "A" 

J 

b 

t^ 

0 

Figs.  133  and  134. — Rectangular  and  Circular 
Columns  with  Respectively  Rectilinear 
and  Curvilinear  Laterals. 

one  and  a  half,  the  cross  section  of  the 
pillar  must  be  sub-divided  by  cross  ties, 
and  the  number  of  vertical  rods  shall  be 
such  that  the  distance  between  the  rods 
along  the  longer  side  of  the  rectangle  shall 
not  exceed  the  distance  between  the  rods 
on  the  shorter  side.  This  is  illustrated  in 
Fig.  133,  where  the  laterals  are  rectilinear, 
and  the  diagram  in  Fig.  134  gives  an  illus- 
tration of  the  use  of  curvilinear  laterals.  In 
the  case  of  concrete  in  columns  in  simple 
compression  the  limiting  stress  is  600  Ib. 
per  square  inch,  and  as  the  steel  and  con- 
crete are  considered  as  being  shortened  in 
the  same  proportion,  the  stress  on  the  steel 
can  never  exceed  m  times  the  stress  on 
the  concrete,  or,  in  other  words,  m  x  c. 
As  m  equals  15,  then  the  permissible  value 
to  allow  for  the  steel  equals  600  x  15  = 
9,000  Ib.  per  square  inch.  In  the  case  of 
columns  which  are  axially  loaded,  there- 


fore, and  when  the  height  does  not  exceed 
eighteen  times  the  least  diameter,  the  safe 
load  will  equal  the  net  sectional  area  of  the 
concrete  in  the  core  multiplied  by  c  plus  the 
sectional  area  of  the  longitudinal  reinforce- 
ment multiplied  by  m  x  c.  As  an  example, 
let  it  be  required  to  find  the  safe  load  that 
can  be  carried  by  a  rectangular  column  15  ft. 
high  which  has  an  effective  size  of  12  in. 
by  12  in.,  and  is  reinforced  with  four  verti- 
cal bars,  each  having  a  diameter  of  1  in. 

The  area  of  one  rod  will  equal  -7854  sq.  in., 
and  the  total  area  of  steel  will  equal  4  x  -7854 

=  3-1416  sq.  in.  The  net  area  of  the  con- 
crete will  equal  (12  x  12)  --  3-1416  = 
144  -  3-1416  =  140-8584  sq.  in.  The  safe 
load  on  the  column  will  equal  A  x  c  +  Ar 

x  m  c,  where  A  equals  the  effective  area  of 
the  concrete,  and  Av  equals  the  sectional 
area  of  the  steel.  Working  this  out  gives 
140-8584  x  600  +  3-1416  x  15  x  600  = 
84515-04  +  28274-4  -  112789-44  Ib.,  which 
equals  just  over  50  tons.  If  preferred,  the 
total  area  of  steel  and  concrete  can  be  taken 
and  multiplied  by  c,  and  to  this  the  extra 
value  of  the  steel  can  be  added  by  taking 
the  sectional  area  and  multiplying  same  by 
fourteen  times  c.  Having  already  con- 
sidered the  area  of  steel  as  taking  600  Ib. 
per  square  inch,  it  will  be  clear  that  the 
extra  value  will  equal  9,000  -  600  Ib.  = 
8,400,  which  is  the  same  as  fourteen  times  c. 
Let  the  previous  example  be  worked  out  in 
this  manner.  Then  the  safe  load  =  144  sq. 
in.  x  600  +  3-1416  x  14  x  600  =  86400  + 
26389-44  =  112789-44  Ib.  as  before.  To 
state  this  as  a  formula,  then  W  the  weight 

=  A  x  c  +  Ay  x  14  c,  or  Ar  x  14  c  =  W 

W  -  (A   x  c). 
-   (A    x  c),  therefore  Av  =  -         ,  . 

J.4:  C 

This  last  formula  will  give  the  method  to 
be  adopted  when  designing  a  short  column 
to  carry  a  certain  given  load,  an  example 
of  which  is  shown.  Design  a  reinforced  con- 
crete circular  column  which  is  12  ft.  high  to 
carry  an  axial  load  of  40  tons.  First  settle 
the  diameter,  which  must  be  at  least  one- 
eighteenth  of  the  height.  If  we  take  9  in., 
this  will  be  one-sixteenth,  and  therefore  well 
within  the  limit.  The  sectional  area  will 
equal  9  x  9  x  -7854  =  63-6174  sq.  in.,  and 
the  load  equals  40  x  2240  =  89,600  Ib. 
89600  -  (63-6174  x  600) 


14  x  600 


51429-56 
"  =      8400™  =        = 


THEORY    OF    REINFORCED    CONCRETE 


119 


rods  are  used,  the  area  of  each  one  will  require 
to  be  1-02  sq.  in.  The  area  of  a  rod  having 
a  diameter  of  lyV  in-  equals  1-1075  sq.  in., 
and  this  size  will  therefore  do  very  well,  and 
the  total  area  of  the  steel  will  equal  1-1075  x  6 
=  6-645  sq.  in.  For  the  purpose  of  practice, 
check  the  column  and  see  what  safe  load  it 
will  carry,  bearing  in  mind  that  the  practical 
area  of  the  steel  equals  6-645  sq.  in.  as 
against  the  theoretical  area  of  6-12  sq.  in.,  and 
the  safe  load  should  therefore  be  slightly 
over  40  tons. 

The  safe  load  equals  A  x  c  +  Av  x  m  c, 

therefore  W  =   { [(9  x  9   x  -7854)  -  6-645] 

x   600}    +  (6-645    x   15    x   600)   =  W  = 

{[63-6174    --   6-645]    x    600 }    +    (6-645    x 

9000) 

W  =  34183-44  +  59805 

W  =  93988-44  Ib.  -  41-5  tons. 
Which  is  correct.     The  column  will  then  be 
as  shown  in  Fig.  134. 

Having  dealt  with  short  columns,  it  now 
remains  to  deal  with  those  known  as  long 
columns,  namely,  where  the  effective  dia- 
meter is  less  than  one-eighteenth  of  the  free 
length.  In  such  columns,  as  previously 
stated,  there  is  a  tendency  for  the  column  to 
fail  by  bending,  although,  theoretically,  there 
should  be  no  such  tendency  if  the  column  is 
absolutely  axially  loaded  ;  but  such  loading 
cannot  be  absolutely  guaranteed  in  practice, 
and  experiments  that  have  been  conducted 
from  time  to  time  with  long  columns  go  to 
show  that  it  is  not  advisable  to  load  the 
column  to  its  full  capacity  as  though  failure 
would  occur  through  direct  crushing.  This 
bending  tendency  cannot  be  measured  or 
calculated  unless  a  certain  definite  amount 
of  eccentricity  is  known,  and  consequently 
an  empirical  formula  has  to  be  used  where 
certain  constants  are  given  deduced  by 
experiment. 

It  is  interesting  to  note  that  the  London 
County  Council  draft  regulations  contain 
conditions  which  limit  the  loading  on  long 
columns,  and  if  the  designer  is  content  to 
accept  the  rules  laid  down  by  such  regula- 
tions, he  will  have  no  need  to  employ  a 
complicated  formula.  The  conditions  are 
given,  as  they  will  no  doubt  be  of  some 
assistance,  and  particularly  in  the  case  of 
checking  the  carrying  capacity  of  a  column 
already  designed.  Briefly  stated,  the  rules 
are  as  follows  : — A  pillar  shall  be  deemed 
to  have  fixed  ends  when  the  ends  of  the 
pillar  are  sufficiently  secured  to  other  parts 


of  the  construction,  having  such  rigidity 
as  will  maintain  the  axis  of  the  pillar  at 
the  ends  in  its  original  vertical  position 
under  all  loads  less  than  the  crippling  load. 
When  both  ends  are  fixed,  and  when  the 
ratio  of  length  to  the  effective  diameter  does 
not  exceed — 

18    The  full  stress  may  be  allowed  on  the 

pillars 

21     -8  of  above  may  be  allowed 
24     -6 
27     -4 
30     -2 

For  other  ratios  the  stress  shall  be  pro- 
portionate to  the  above.  In  the  case  of 
compression  members  not  having  both  ends 
fixed,  the  loads  shall  be  as  follows  :  Let  P  = 
the  maximum  pressure  on  pillars  and  com- 
pression members  having  fixed  ends. 


Condition  of  Ends 


One  end  fixed  and  one  end  hinged 

Both  ends  hinged 

One  end  fixed  and  the  other  end 
free  and  not  guided,  stayed  or 
supported  in  all  directions  . . 


Working 

load 


_P 

2 
P 
4 

P 

16 


Unfortunately,  there  is  a  great  difference 
between  the  various  formulse  that  have 
been  deduced  by  various  authors,  and  if  the 
student  were  calculating,  say,  a  wooden 
strut,  and  he  adopted  Gordon's  formulse,  and 
checked  the  result  by  using  Bitter's  formula, 
he  would  find  that  the  latter  would  give  a 
load  which  would  be  about  five  times  as 
great  as  that  found  by  Gordon's.  Thus, 
there  is  no  real  proof  as  to  what  safe  load 
can  be  adopted,  or  such  a  variation  would 
not  be  possible,  and  the  most  that  can  be 
done  is  to  recommend  the  student  to  adopt 
a  fairly  safe  method.  Gordon's  formula  has 
already  been  given  in  the  preceding  chapter  ; 
it  is  one  of  the  best  known  in  this  country, 
and  is  greatly  used.  In  the  absence  of  exten- 
sive experiments  with  long,  reinforced  con- 
crete columns,  this  formula  may  be  adapted 
for  designing  such  columns,  as  hereafter  ex- 
plained. The  formula  is  as  follows  :  Rc  = 

Ar 

c—r2,  where   Rc    =   the    safe    load    in 

1  +  a^ 

the  same  units  as  rc,  A   =  the  area  of  the 


120 


REINFORCED    CONCRETE 


cross  section,  rc  =  the  safe  resistance  of  the 
material  in  compression,  a  is  a  constant 
derived  by  experiment,  I  is  the  length  in  the 
same  units  as  d,  and  d  is  the  effective  dia- 
meter. It  will  be  seen  that  the  value  Arc 
would  equal  the  safe  resistance  if  the  column 
were  a  short  one,  and  consequently  the 

I2 
value  1  +  a  -ft  is  the  reducing  factor  which 

is  dependent  on  the  ratio  of  length  to  dia- 
meter, and  also  upon  a,  which  is  governed 
by  the  conditions  of  fixing.  In  the  table 
given  on  p.  79  it  will  be  seen  that  for 
wrought-iron  and  steel,  where  the  section 
was  rectangular  or  circular,  the  values  for 

4 
a    were    as    follow  :      Ends    hinged    ^v 


ends   fixed 


one   hinged  and  one      •&-„  — 


fixed  jTyyv  an<^  these  same  constants  may  be 


taken.  As  the  value  A  rr  in  this  formula 
equals  the  safe  resistance  of  a  short  column, 
this  can  be  substituted  by  the  expression 
which  is  already  deduced  for  giving  this 
value,  namely,  A  x  c  +  Av  x  me,  and  if 
the  reducing  factors  given  above  are  applied, 
the  complete  formula  becomes 

A  x  c  +  Av  x   me 

W    =  /2 

l+(ld* 

and  this  must  be  used  when  checking  the 
weight-carrying  capacity  of  a  column  which 
has  a  diameter  less  than  one-eighteenth  of 
the  free  length. 

In  the  case  of  designing  a  short  column, 
it  was  shown  that  the  area  of  steel  re- 
quired was  calculated  by  the  formula  Av  = 
W  -  (A  x  c) 

—  j,       —  ,  where  A  =  the  total  area  of 

the  cross  section.     It  will  now  be  obvious 

Z2 
that  the  value  1   x  a  ^  must  again  be  in- 

troduced, and  more  steel  will  be  required 
than  would  be  the  case  with  a  short  column. 
The  actual  weight  must  therefore  be  multi- 
plied by  this  value,  giving  a  formula  as 
follows  : 


W 


Av  = 


(12\ 
1  +  a  -p  ]  —  (A  x  c) 
/ • 


14  c 


Having  now  deduced  formulae  for  finding  the 
area  of  steel  required,  and  also  for  checking 
the  column  to  see  the  safe  load  which  it 
can  carry,  the  application  of  these  will  be 


shown  in  an  example.  A  column  is  fixed  at 
the  ends,  and  has  a  free  length  of  20  ft., 
while  the  load  to  be  carried  is  50  tons.  The 
column  is  to  be  rectangular,  and  it  is  limited 
in  size  to  12  in.  square.  Calculate  the 
amount  of  vertical  reinforcement  required, 
and  afterwards  check  the  load  the  column 
as  designed  would  carry.  The  size  given  is 
the  effective  size,  and  the  load  is  assumed 
to  include  the  weight  of  the  column  itself. 
The  load  equals  50  tons,  which  is  equivalent 
to  112,000  Ib.  The  effective  sectional  area 
will  equal  12  x  12  =  144  sq.  in.,  and  as  the 

ends  are  fixed,  the  value  of  a  will  be 
The  steel  will  be  found  by  the  formula  : 
W  x  (l  +  aj[)-  (A  x  c) 


112000  x    1 + 


14  c 
J_ 

2500 


— ,  therefore 

2402\ 
x-Tp-j-  (144x600) 

/ 


A,  = 

Av  - 


14  x  6CO 
112000  x  (1  +  -16)  -  86400 

8400 
129920  -  86400 


=  A,  = 


43520 

8400 


8400 
Av  =  5-18  sq.  in. 

If  four  bars  are  used,  then  each  will 
require  an  area  of  1-295  sq.  in.  A  round 
bar  with  a  diameter  of  1T^-  in.  has  an  area 
of  1-353  sq.  in.,  and  this  is  the  nearest  size 
obtainable,  and  the  total  area  of  steel 
required  will  therefore  be  1-353  x  4  =  5-412 
sq.  in.  As  designed,  therefore,  the  column 
is  20  ft.  high,  fixed  at  the  ends,  12  in.  square 
effective  size,  and  reinforced  with  four  1T"F 
in.  diameter  bars.  This  should  next  be 
checked  by  calculating  the  safe  load  it  will 
carry,  and,  to  be  correct,  this  load  must 
not  be  less  than  50  tons.  Now 
A  xc+A,,  x  me 


W  - 


therefore 


W  = 


W  - 


138-588   x  600  +  5-412   x  15   x  600 


1   + 


2402 


25CO 
83152-8  +  48708 


W  - 


131860-8 


1-16  1-16 

W  =  113673  Ib.  =  50-7  tons,  which  is  quite 
satisfactory,  the  slight  amount  over  50  tons 
being  due  to  the  fact  that  the  actual  amount 
of  steel  equals  5-412  sq.  in.,  which  is  slightly 
in  excess  of  the  theoretical  amount  calcu- 
lated, namely,  5-18  sq.  in. 

In  order  to  form  a  comparison  with  the 


THEORY    OF    REINFORCED    CONCRETE 


121 


conditions  given  in  the  draft  regulations  of 
the  London  County  Council,  as  previously 
stated,  it  will  be  interesting  to  calculate  what 
load  could  be  carried  by  this  column  if  the 
rules  given  therein  are  applied.  In  the  first 
instance,  it  will  be  necessary  to  work  out  the 
load  that  could  safely  be  put  on  a  short 
column  having  the  same  sectional  area  and 
reinforcement,  and  then  apply  the  reducing 
factor  which  is  dependent  on  the  ratio  of 
diameter  to  length,  as  given  in  the  table 
previously. 

The  safe  load  for  a  short  column  would  be 
found  by  the  formula  : 
W  =  A  x  c  +  Av  x  m  c,  therefore 
W  =  138-588  x  600  +  5412  x  15  x  600 
W  =  83152-8  +  48708  =  W  =  131860-8  Ib. 
This,  then,  would  equal  the  safe  load  for  a 
short  column,  and  the  ratio  of  diameter  to 
length  must  next  be  considered.  The  length 
is  given  as  20  ft.,  and  the  effective  diameter 
equals  12  in.,  and  the  ratio,  therefore,  equals 
20.  Upon  referring  to  the  table,  it  will  be 
seen  that  where  the  ratio  of  diameter  and 
length  does  not  exceed  21,  the  safe  load  can 
be  taken  as  -8  of  that  for  a  short  column. 
The  safe  load  for  this  example  will  therefore 
equal  131860-8  Ib.  x  -8,  which  equals 
105488-64:  Ib.,  or  47-09  tons.  This  is  slightly 
less  than  the  safe  load,  as  calculated  by  the 
formula,  which  was  50-7  tons  and  when 
working  in  actual  practice,  it  would  be  neces- 
sary to  keep  within  the  limits  allowed  by  the 
London  County  Council  rules,  if  within  the 
County  of  London.  It  must  be  remem- 
bered, also,  that  the  ratio  of  length  to  dia- 
meter is  actually  20,  and  not  21  as  taken, 
and  this,  obviously,  will  affect  the  result. 
It  is  very  seldom,  however,  that  such  a  fine 
line  would  be  drawn,  and  the  designer  in 
calculating  the  loads  c  nnot  work  so  closely 
to  the  actual  loads  as  to  give  no  margin 
whatever.  As  stated  previously,  the  vari- 
ous formulae  and  methods  for  calculating 
compression  members  will  always  be  found 
to  vary  considerably,  and  in  this  instance 
there  is  only  a  difference  of  about  6  per  cent. 

COLUMNS   ECCENTRICALLY    LOADED 

It  is  proposed  to  give  a  few  notes  on  the 
eccentric  loading  of  columns  and  its  effect, 
although  the  value  of  the  eccentricity  is 
very  difficult  to  obtain,  unless  such  eccen- 
tricity is  caused  by  certain  definite  loads 
applied  at  a  certain  definite  distance  from 
the  centre  of  the  column.  Eccentricity  in 
loading  is  caused  in  many  ways,  a  common 


case  being  that  where  the  load  from  a  beam 
is  transmitted  through  a  bracket  projecting 
from  the  column,  or  by  the  beam  not  being 
carried  right  over  the  column.  The  eccen- 
tricity is  sometimes  such  that  it  causes  one 
edge  of  the  column  to  be  in  tension,  and 
sufficient  reinforcement  must  then  be  pro- 
vided to  resist  same.  The  fact  of  the  load 
not  being  axial  will  cause  the  stress  in  the 
column  to  vary  over  the  area  of  the  cross 
section,  and  the  compression  will  be  at  its 
maximum  at  that  edge  which  is  the  nearest  to 
the  eccentric  load,  and  at  a  minimum  at  that 
edge  which  is  the  farthest  away  from  same. 
It  is  necessary,  therefore,  to  consider  the 
two  extremes,  and  this  can  be  done  by  find- 
ing the  pressure  that  would  occur  if  the 
load  were  axial,  and  adding  or  deducting  the 
value  of  the  eccentricity.  Where  the  com- 
pression is  at  a  maximum,  therefore. 


Max.  c  = 


W       Wy 


AE         Z 

where  W  =  the  load,  AE  =  the  equivalent 
area  of  the  concrete  in  square  inches  =  the 
sectional  area  of  the  concrete  +  14  times  the 
sectional  area  of  the  steel  =  A  +  (w  -  1)  A,,, 
y  =  the  eccentricity  of  the  load  in  inches, 
or,  in  other  words,  the  distance  from  its 
point  of  application  to  the  neutral  axis  of 
the  section,  and  Z  =  the  section  modulus. 
This  last  expression  was  explained  in  the 
preceding  chapter,  and  the  following  values 
for  reinforced  concrete  columns  are  given. 

d  2 
Eectangular  Z  =^A.d  +  ^(m-  I)  Av  -4-, 

where  A  =  the  total  area  of  the  cross  section, 
d  =  the  outside  diameter  of  the  column,  and 
d±  =  the  distance  from  centre  to  centre  of 
the  vertical  reinforcement.  Circular  column 
with  four  bars  only 

d£ 

Zj    • —    a"  .A.  (t    "T   ??  \)Tl         J. )    -i*-«      ^~, 

d  ' 

and  for  circular  column  with  bars  arranged 
in  a  circle, 

On  that  edge  farthest  from  the  load  the  mini- 

W         W?/ 
mum  compression  =  Mm.  c  •-    — r- =-. 

From  the  above  formula  may  be  deduced  a 
formula  for  finding  W,  which  is 
Max.  c  x  AE  x  Z 


W  = 


Z  +  AE  x  y 


It  will  now  be  advisable  to  work  out  an 
example  to  show  the  application  of  these 


122 


REINFORCED    CONCRETE 


formulae.  Find  the  safe  load  that  can  be 
carried  by  a  column  16  in.  square  reinforced 
with  eight  IJ-in.  rods,  the  load  being  applied 
vertically  at  a  distance  of  2  in.  from  the  axis, 
and  the  distance  from  centre  to  centre  of 


Fig.   135.   Designing  Column  Eccentrically 
Loaded 


rods  being  13  in.  The  maximum  stress  on 
the  concrete  is  not  to  exceed  500  Ib.  per 
square  inch.  The  plan  of  the  column,  as 
given,  is  shown  in  Fig.  135,  and  upon  refer- 
ence to  this  the  symbols  can  be  followed. 
The  safe  load  will  be  found  according  to  the 
formula  : 


W  = 


Max.  c  x  A  x  Z 


^  +  AE  x  y 
Max.  c  is  given  as  500  Ib.,  and  y  =  2  in., 
while  AE  will  equal  A  +  (m  -  1)  Av  =  16  x 
16  +  (15  -  1)  9-8176  =  256  +  137-446  = 
AE  =  393-445.  Av  is  given  by  the  area  of 
the  rods,  the  area  of  one  IJ-in.  rod  =  1-2272 
sq.  in.,  and  the  total  area  =  1-2272  x  8  = 
9-8176  sq.  in.,  as  given  above.  The  value 
of  Z  must  next  be  found  by  the  expression 

Z   =  (£  A    x    d)  +  %  (m  -  1)  Av    x   -i 

therefore, 

Z  =  (i  x  256  x  16)  +  \  x  14  x  9-8176  x  •- -- 

Z  =  682-66  +  (68-7232  x  10-56) 
Z  =  682-66  +  725-717 
Z  =  1408-377. 

Having  determined  the  value  of  the  vari- 
ous factors,  proceed  to  find  the  value  of  the 
weight  as  follows  : — 

...      500  x  393-446  x  1408-377 
W  = 


W  = 


1408-377  +  393-446  x  2 
277060148-571 


As  a  check  to  this,  and  also  as  an  example 
of  the  use  of  the  formulae  for  the  maximum 
and  minimum  compression,  let  it  be  required 
to  calculate  the  maximum  and  minimum 
compression  on  the  column  above  men- 
tioned, and  carrying  the  calculated  eccentric 
load.  Then  the  maximum  compression, 
which  should  not  exceed  500  Ib.,  will  be 
found  by  the  formula  : 

W       W  y 
Max.  c  =  -j-  H — —. 

The  values  of  AE  and  Z  have  already  been 
calculated,  and  therefore, 

126207       126207  x  2 
~  393-446  4     1408-377 
Max.  c   =  320   +   179-2    ==  499-2  Ib., 

which  is  within  the  limit.  The  minimum 
compression  will  be  given  by  the  expres- 
sion : 


Min.  c  = 


126207       126207  x  2 


393446         1408-377 
Min.  c  =320  -  179-2  =  140-8  Ib. 

The  edge  of  the  column  farthest  from  the 
load  is  therefore  not  in  tension,  and  the 
column  is  satisfactory.  The  diagram  given 
in  Fig.  136  shows  how  the  stress  varies  across 
the  section,  and  the  length  of  the  vertical 
line  at  any  intermediate  point  represents  the 


2195-269 
W  =  126207  Ib.,  or  just  over  56  tons. 


-  499'2ia» 


Fig.   136. — Stress  in  Column  Eccentrically 
Loaded 


compression  stress  at  that  point.  The  stress 
on  the  steel  being  fifteen  times  that  on  the 
concrete  the  value  can  be  found  in  any  of  the 
bars  by  scaling  the  length  of  the  line  coinci- 
dent with  the  axis  of  the  bar,  and  mul- 
tiplying same  by  15. 


THEORY    OF    REINFORCED    CONCRETE 


123 


RETAINING   WALLS 

It  is  not  proposed  in  these  notes  to  cover 
the  whole  theory  and  design  of  retaining 
walls,  but  rather  to  give  some  general  re- 
marks to  enable  the  student  to  form  an  idea 
of  the  principles  governing  the  design  of  such 
structures.  Walls  may  be  used  to  retain  or 
support  either  earth  or  water,  and  the 
method  employed  in  reinforced  concrete  is 
very  different  from  that  which  is  customary 
with  brickwork  masonry  or  plain  concrete. 


Fig.  137. — Influence  of  Angle  of  Repose 
and  Line  of  Rupture  on  Design  of 
Retaining  Walls 

In  the  last-mentioned  case  the  wall  is  depen- 
dent on  its  thickness  and  weight,  which  must 
be  such  that  its  gravity  is  not  overcome  by 
the  pressure  acting  on  the  back  of  the  wall ; 
whereas  in  the  case  of  reinforced  concrete 
the  wall  is  designed  to  offer  sufficient  resist- 
ance by  its  strength.  Tension  at  the  back 
of  a  reinforced  concrete  wall  is  permissible, 
and  steel  is  provided  to  resist  it ;  but  it  is 
obvious  that  a  brick  wall,  for  example,  would 
be  capable  of  resisting  very  little  tensional 
stress.  A  good  comparison  of  the  two 
methods  of  design  is  offered  by  the  front 
and  back  retaining  walls  constructed  at  the 
Royal  Automobile  Club  (described  in  a  later 


chapter),  where  both  plain  and  reinforced 
concrete  were  used. 

In  dealing  with  walls  to  retain  earth,  it  is 
necessary,  in  the  first  instance,  to  investi- 
gate the  nature  and  value  of  the  force  that 
has  to  be  resisted.  It  will  be  quite  clear 
that  a  mass  of  earth  standing  alone  without 
lateral  support  will  not  remain  with  vertical 
faces,  but  that  some  of  its  particles  will  fall 
past  one  another,  since  they  have  little  or 
no  cohesion,  and  form  a  mound  having  slop- 
ing surfaces.  Sand  poured  upon  a  floor  forms 
a  mound  having  sloping  sides,  and  it  will 
be  seen  that  there  is  a  certain  angle  with 
the  horizontal  which  the  material  will  main- 
tain even  without  lateral  support.  The 
angle  depends  on  the  friction  between  the 
particles,  and  is  called  the  "  angle  of  repose  "; 
it  varies  in  different  materials,  and  also  in 
the  same  material  according  to-  the  state  of 
consolidation  and  dryness.  The  following 
table  must  be  taken  merely  as  a  guide,  and 
cannot  be  considered  as  fixed  by  any  hard- 
and-fast  rule  : — 


Material 


Wet  clay  or  vegetable  earth 

Wet  sand 

Dry  clay,  sand,  or  vegetable  earth 

Sandy  gravel 

Clean,  firm  gravel 

Loose  shingle 

Hard,  dry  vegetable  earth 

Clay,  well  drained  . . 


16C 

20C 
30C 
26° 
45C 
40° 
45C 
45° 


This  angle  can  be  considered,  then,  as  the 
natural  angle  at  which  the  material  will 
rest,  and  this  is  shown  in  diagram  A  (Fig. 
137),  where  the  line  of  rupture  is  also  indi- 
cated. It  is  considered  that  only  the  wedge- 
shaped  portion  of  earth  contained  between 
the  line  of  rupture  and  the  face  of  the  wall 
will  require  to  be  supported,  as  upon  the 
sudden  removal  of  the  wall  it  is  that  portion 
which  would  slip,  and  this  has  been  proved 
more  or  less  by  actual  experience.  There  will 
also  be  a  large  amount  of  friction  set  up 
between  the  portion  that  is  tending  to  slide 
and  the  portion  that  is  normally  at  rest,  and 
by  considering  that  portion  above  the  line  of 
rupture  only,  an  allowance  is  made  for 
this,  and  it  will  be  seen  in  the  diagram  that 
the  wedge-shaped  portion  in  tending  to  slip 
downward  is  really  sliding  on  the  inclined 
plane  represented  by  the  line  of  rupture. 

The  line  of  rupture  is  found  by  setting 
out  the  angle  of  repose  and  bi-secting  the 


124 


REINFORCED    CONCRETE 


angle  between  this  and  the  back  of  the  wall ; 
the  earth  that  is  contained  in  this  triangle 
is  calculated  and  the  load  worked  out 
according  to  the  weight  per  cubic  foot  of 
the  particular  earth  under  consideration. 
It  is  usual  to  take  a  length  of  1  ft.  in  the 
calculations,  both  in  calculating  the  weight 
of  the  soil  to  be  supported  and  in  the  design 
of  the  wall.  In  the  calculations  it  is  neces- 
sary to  consider  both  the  vertical  portion 
of  the  wall  and  also  the  base,  which  requires 
to  be  sufficiently  large  to  distribute  the 
pressure  over  such  an  area  of  foundation 
as  to  bring  the  load  per  square  foot  within 
the  safe  resistance  of  the  soil.  Also  the 
base  must  be  of  sufficient  thickness  to  allow 
the  projecting  portions  to  act  as  cantilevers. 
The  vertical  portion  will  be  a  cantilever 


on  the  back  of  the  wall  will  be  found  by 
drawing  a  line  parallel  to  the  line  of  rupture 
from  the  top  of  the  line  representing  the 
weight  until  it  cuts  the  horizontal  line  drawn 
through  D.  This  horizontal  thrust  can  then 
be  multiplied  by  its  distance  from  the 
point  of  intersection  with  the  base,  which 

will  equal  -,  and  this  will  give  the  bending 
o 

moment  to  be  resisted.  The  bending  moment 
will  gradually  diminish  towards  the  top, 
where  it  will  be  nil. 

With  regard  to  the  base,  there  are  three 
general  types  of  wall  employed,  as  illus- 
trated at  A,  B,  and  C  (Fig.  138),  the  selec- 
tion of  one  of  which  will  depend  upon  cir- 
cumstances. The  type  shown  at  A,  for 
example,  could  not  be  employed  if  the 


/////'/ 

V//  ///////' 
tAEJH 

'/     \ 

'/     N 

1  • 

\  fCovnjvs.fOK\ 
1 

\ 

\ 

\ 

y 

\ 

'/, 

\ 

'//,  A 

\ 

'/     EAKTO 


B 
Fig.   138.— Three  General  Types  of  Retaining  Walls. 


having  a  length  equal  to  the  height  of  the 
wall,  and  it  is  obvious  that  the  greatest 
stress  will  occur  at  the  bottom,  and  this  will 
be  due  to  the  outward  pressure  of  the  earth 
which  can  be  ascertained  as  shown  in  dia- 
gram B  (Fig.  137),  when  the  centre  of  pres- 
sure of  the  triangular  prism  of  earth  ABC 

is   shown  at  a  point  equal   to  ---   from  the 

o 

bottom,  where  h  equals  the  height  of  the 
wall.  The  value  of  the  pressure  will  be 
found  by  calculating  the  weight  of  the  earth 
contained  in  the  triangle,  and  also  the  centre 
of  gravity  of  the  triangle  as  illustrated. 
The  weight  will  act  downward  through  this 
centre  of  gravity,  and  if  a  line  is  drawn  down- 
ward through  this  point  until  it  cuts  the 
line  of  rupture  at  D  as  shown,  and  the 
amount  of  the  weight  is  set  up  to  scale  from 
this  point,  the  horizontal  equivalent  thrust 


adjacent  earth  was  the  property  of  an 
adjoining  owner  who  might  raise  objection 
to  the  projection  under  his  land.  This  was 
the  case  at  the  Koyal  Automobile  Club,  and 
it  became  necessary  to  design  the  wall  with 
the  projecting  toe  extending  into  the  build- 
ing as  illustrated  at  B.  In  the  former  case 
the  weight  of  the  earth  acting  downward 
on  to  the  base  will  tend  to  prevent  the  over- 
turning of  the  wall,  whereas  in  B  this  is 
not  the  case,  and  sufficient  weight  must  be 
provided  to  prevent  the  wall  from  being 
thrust  over  on  the  extreme  outer  point  of 
the  toe,  which  will  be  the  fulcrum  of  the 
lever.  The  type  shown  at  C  is  some- 
what between  the  others,  having  a  projec- 
tion on  both  sides.  Provision  to  prevent 
sliding  on  the  base  is  often  made  by  step- 
ping the  under  side  in  such  manner  that 
vertical  surfaces  are  provided,  as  shown 


THEORY    OF    REINFORCED    CONCRETE 


125 


by  dotted  lines  in  B.  The  length  of  the  base 
for  preliminary  calculations  is  often  taken 
as  about  one-half  of  the  height,  and  the 
thickness  is,  of  course,  calculated  from  the 
bending  moment,  which  it  has  to  resist  as  a 
cantilever. 

The  type  shown  at  A  is  often  stiffened  by 
counterforts  at  intervals,  as  shown  by  dotted 
lines,  these  serving  as  stiffeners,  both  to 
the  vertical  wall  and  the  base.  In  London 
work  it  is  often  necessary  to  construct 
vaults  under  the  pavement,  when  the  outer 


Fig.    139. — Retaining  Wall  with  Cantilevers 
under  Footpath 

wall  will  act  as  a  retaining  wall  to  support 
the  roadway.  A  type  that  is  sometimes 
employed  under  such  circumstances  is  illus- 
trated in  Fig.  139,  where  the  top  horizontal 
portion  acts  as  a  cantilever  from  the  vertical 
wall  and  supports  the  footpath,  while  the 
base  acts  as  a  cantilever  from  the  bottom 
of  the  wall  and  has  to  resist  the  earth  pres- 
sure. This  type  of  construction  was  em- 
ployed at  Messrs.  Whiteleys'  new  building 
in  Queen's  Road,  Bayswater,  London. 

In  the  case  of  walls  called  upon  to  resist 
the  pressure  of  water,  matters  are  somewhat 
simplified,  as  there  is  no  variation  due  to 
weight  or  angle  of  repose,  and  the  pressure 
will  increase  according  to  the  depth  of  the 
water  and  always  act  at  right  angles  to  any 


surface.  A  cubic  foot  of  water  weighs  62£  lb.^ 
and  the  pressure  in  pounds  per  foot  against 
the  wall  will  therefore  equal  the  depth  of  the 
water  in  feet  multiplied  by  62|.  It  will 
be  seen,  therefore,  that  the  pressure  varies 
directly  as  the  depth  as  shown  by  the  shaded 
portion  in  Fig.  140,  and  the  centre  of  the 
whole  pressure  will  be  situated  at  the  centre 
of  gravity  of  the  triangle,  which  will  be  situ- 
ated at  one-third  the  height  from  the  bottom. 
The  value  of  this  pressure  will  equal  the 
height  multiplied  by  the  mean  pressure,  the 

62|  x  height 
latter  being  =  -    — - —    — .     This  pressure 

jt 

multiplied  by  the  height  from  the  intersec- 
tion of  the  wall  with  the  base  will  give  the 


Fig.   140.— Retaining  Wall  to  Resist  Water 
Pressure 

maximum  bending  moment.  In  the  case  of 
walls  where  the  surface  next  the  water  i& 
sloping,  the  total  pressure  will  be  greater  or 
less  according  to  whether  the  surface  is 
sloping  away  from  or  to  the  water  ;  but 
as  the  pressure  will  always  act  at  right 
angles  to  the  surface,  the  variation  in  the 
angle  of  the  pressure  will  compensate  for 
any  variation  in  the  amount  of  the  pressure. 
The  subject  of  retaining  walls  is  one  that 
requires  very  careful  consideration  when 
designing,  and  the  student  is  advised  to 
acquire  a  knowledge  of  trigonometry  before 
attempting  to  become  fully  conversant  with 
this  branch  of  reinforced  concrete  work. 
A  study  of  the  examples  already  carried  out 
in  practice  will  also  be  helpful. 


The    Erection    of    a    Reinforced 
Concrete    Building 


IN  tliis  chapter  will  be  described  tlie  method 
or  system  of  carrying  out  the  construction 
of  the  complete  carcass  of  a  reinforced  con- 
crete building.  Attention  will  be  drawn, 
not  only  to  the  correct  way  of  doing  the 
work,  but  also  to  the  faults  and  pitfalls 
that  have  to  be  guarded  against. 

In  reinforced  concrete  work  there  are  four 
things  that  need  to  be  most  carefully 
observed  :  (1)  strict  adherence  to  the  special- 


and  efficiency  of  the  structure  depend,  and 
if  they  are  not  most  strictly  observed  failure 
is  almost  inevitable.  Failure,  be  it  always 
remembered,  not  only  means  great  and 
unnecessary  expense,  but  involves  enormous 
risk  to  life  and  limb.  It  need  hardly  be 
said  that  it  behoves  all  those  who  undertake 
reinforced  concrete  construction  to  employ 
only  reliable  supervision  and  labour. 


1st  engineer's  drawings  as  to  the  deposition      BUILDING  A  FACTORY 
of  the  steel  reinforcements  ;    (2)  careful  and         It  will  be  assumed  that  the  building  to  be 
efficient  mixing  and  punning  of  the  concrete      erected  is  of  the  warehouse  or  factory  type, 
being  some  three  or   four   stories  in 
1  ^j         height  (see  Fig.  141),   and   to   which 

1  !  

!  .  !  

will  be  attached  the  usual  engine  and 
boiler-house,    generally    a    one-story 
structure.     It   will   also    be    assumed 
that  the  site  is  a  bad  one,  and  the 
engineers  have  found  it  necessary  to 
provide    a    reinforced    concrete    pile 
foundation  for  the  main  building,  and 
a  reinforced  concrete  raft  for  the  en- 
gine and  boiler-house.     To  the  ware- 
house there  will  be  a  large  basement, 
which  will   involve  the  erection  of  a 
retaining  wall    to    support  the  road- 
way   on    the 
frontage. 

•         • 

I               '                " 

1                 ' 

•         • 

1               "               ' 

'                 ' 

-dllllLJUN—,  \           Ti,   Carting 

• 

,  !  !  

1     '-  !.  

' 

a  job  of  this 
description, 
^                                              |     the  first  thing 

] 

,^^-»fl.->-"m«.ilnvii^,""'  "4™- 

I              J 

Fig.   141.—  Vertical 

1          11 

I     0 

Cross  Section 

( 

m»O^.  i'-*1^"^  VWWWjdyj 

1  .    ( 

of  Typical  Fac 
Concrete 

rsite  of  all  rub- 
bish, hack  up 
all  old  found 
ations,      and 
»                                                  leave  the  site 
tory  Building  in  Reinforced            clear  for  im- 
mediate oper- 
ations. Next, 

so  that  the  steel  is  entirely  surrounded  by 
this  material ;  (3)  the  proper  design  and 
the  complete  stability  of  the  falsework ; 
(4)  the  proper  and  careful  striking  of  the 
falsework. 

On  these  items  the  strength,  durability, 


the  general  foreman  must  select  suitable 
positions  for  his  office  and  the  office  of  the 
clerk  of  works,  and  for  the  building  material 
stores  ;  for  the  last -mentioned  it  will  be 
necessary  to  erect  some  sheds.  The  con- 
tractor is  now  ready  to  receive  materials. 


126 


ERECTION    OF    A    BUILDING 


127 


Receiving  the  Bars. — Considering  first 
the  arrival  of  the  steel  reinforcement  rods, 
these  should  be  unloaded  carefully  so  that 
the  thinner  rods  are  not  twisted  ;  they  are 
easily  put  out  of  shape  by  rough  handling. 
All  the  rods  and  bars  must  be  sorted  and 
bundled  in  their  respective  lengths  and 
diameters,  a  complete  list  of  which  must 
be  kept  by  the  foreman.  The  sizes  should 
be  indicated  by  pegs  or  in  some  other  way, 
so  that  the  risk  of  errors  may  be  lessened  ; 
the  importance  of  this  will  be  realised  when 
it  is  remembered  that  a  unit  of  one-sixteenth 
of  an  inch  is  adopted  in  specifying  the  bars, 
and  should  bars  be  in  a  wrong  place  or  of  a 
smaller  diameter  than  detailed  trouble  may 
be  caused.  Bars  that  arrive  twisted  and  dis- 
torted must  be  put  aside  to  be  straightened. 

Steel  bars  are  frequently  laid  down  in  the 
open,  but  this  is  not  good  practice,  and 
temporary  sheds  should  be  erected  for  their 
reception,  because  exposed  steel  becomes 
badly  scaled  and  pitted  with  rust,  which 
ought  not  on  any  account  to  be  permitted. 
Though  it  has  been  recognised  that  slight 
rust  is  beneficial  for  the  protection  of  steel 
in  concrete,  deep  pitting  or  rusting  means 
the  reduction  of  the  diameter  appreciably. 
The  amount  of  rust  permissible  will  occur 
while  the  steel  is  lying  in  a  temporary 
building.  Where  there  is  no  room  for 
sheds,  the  steel  should  be  given  a  coat  of 
cement  wash  and  then  covered  with  tar- 
paulins. It  is  a  good  practice  to  erect 
temporary  racks  or  stands  in  the  sheds  for 
the  reception  of  the  bars,  so  that  they  may 
be  easily  lifted  down  when  required  ;  this 
method  will  prevent  excessive  distortion  in 
bars  less  than  f  in.  in  diameter,  which  get 
very  badly  twisted  when  laid  in  rough  heaps 
of  considerable  weight ;  the  wholesale 
straightening  of  bars  should  be  unnecessary. 
At  the  end  of  each  division  in  the  steel  rack 
a  board  should  be  attached  with  the  lengths 
and  diameters  of  the  rods  indicated  thereon, 
as  in  the  accompanying  table,  the  numbers 
in  the  first  column  indicating  the  number  of 


1  in. 

TV  in. 

l|in. 

50    5—0 
50    5—6  ! 
35    6—9 
42    9—6 
82   11—6 
95   20—6 

120   10—0 
128   12—6 
141   26—6 
163   27—6 
181   30—0 
200  31—6 

32   14—0 
38   16—6 
58   17—9 
64   18—6 
73   19—0 

80   20—0 

• 

bars  in  stock,  and  those  in  the  second  giving 
the  lengths  in  feet  and  inches. 

By  this  simple  means  any  bar  of  any  par- 
ticular length  can  be  obtained  at  once,  which 
is  impossible  when  all  bars  of  the  same 
length  are  bundled  together  irrespective  of 
their  thickness. 

Aggregate  and  Sand. — The  position 
allotted  for  the  deposit  of  the  gravel  and 
sand  should  be  as  near  the  mixing  stage  as 
possible,  so  as  to  save  long  barrow  runs.  In 
most  cases  the  sand  and  gravel  come  to  the 
job  already  screened  and  washed,  and  in  a 
large  contract  this  is  nearly  always  advisable, 
as  otherwise  a  gang  of  men  would  have  to 
be  kept  continually  washing  and  screening. 
Should  the  contractor  be  fortunate  enough 
to  find  sand,  etc.,  on  the  site,  the  washing, 
etc.,  will  be  inevitable.  The  gravel  or 
aggregate  must  be  deposited  in  a  separate 
heap  from  the  sand.  Both  the  clerk  of 
works  and  the  foreman  should  see  that  there* 
is  no  clay  or  loam  mixed  with  the  sand  or 
gravel.  Should  broken  stone  or  broken 
brick  constitute  the  aggregate,  see  that  it  is 
free  from  dust ;  in  the  case  of  furnace  slag 
or  similar  material,  no  sulphur  or  other 
impurities  should  be  present.  In  regard  to 
sand,  sharp  coarse  river  sand  is  preferable, 
or  good  pit  sand  of  various  size  grains  up 
to  particles  that  will  pass  through  a  J-in. 
square  mesh,  and  of  which  at  least  75  per 
cent,  will  pass  through  a  J-in.  square  mesh. 
Sand  must  be  free  from  all  ligneous  (woody), 
organic  or  loamy  substances. 

Cement. — For  the  cement  a  specially  dry 
shed  should  be  erected.  The  familiar  rough 
sheds,  which  are  not  proof  against  weather, 
should  not  be  permitted.  As  to  the  testing 
of  the  cement,  that  is  fully  dealt  with  else- 
where in  this  book  ;  but  it  should  be  said 
here  that  the  foreman  and  the  clerk  of 
works  should  see  that  the  cement  arrives  in 
sealed  bags  bearing  the  maker's  name,  and 
they  should  from  time  to  time  make  prac- 
tical tests  to  see  that  the  cement  is  cool 
and  in  perfect  condition.  Cement  must 
always  be  stored  out  of  contact  with  wind, 
damp  air,  damp  walls,  or  damp  ground,  as 
otherwise  it  quickly  loses  its  strength. 

Water. — If  practicable,  water  should  be 
supplied  from  a  main,  but  if  not,  it  should 
come  from  a  good  spring  or  boring.  It  is 
best  to  have  the  water  laid  on  by  a  supply 
pipe  to  the  mixing  stage,  and  such  a  supply 
is  a  necessity  when  machine  mixing — 
which  is  here  advocated — is  adopted. 


123 


REINFORCED    CONCRETE 


Mixing  Stage. — The  foreman  must  con- 
sider carefully  the  position  of  the  mixing 
stage  or  the  mixing  machine,  as  it  is  from 
this  point  that  the  building  grows.  Prefer- 
ably the  stage  should  always  be  open  to 


Fig.    142.— Mixing  Stage 

inspection  throughout  the  entire  period  of 
the  job  ;  certainly  it  should  not  be  hidden 
in  an  obscure  corner,  where  it  is  difficult  to 
watch  and  check  the  operations.  The  ideal 
position  for  a  mixing  stage  is  one  where  it 
can  remain  as  long  as  possible  and  where 
it  is  convenient  for  the  hoisting  of  the  con- 
crete to  the  different  parts  of  the  building. 


at  each  end  to  form  handles  (see  Fig.  142). 
A  2-in.  fillet  nailed  round  the  board  will 
prevent  the  wastage  of  cement.  A  level 
space  should  be  prepared  for  the  mixing 
board. 

Hoisting. — Provision  should  be  made  for 
hoisting  the  concrete,  timber  and  steel. 
On  a  large  job  a  derrick  and  Scotchman 
should  be  erected,  as  well  as  a  concrete 
hoist,  some  efficient  types  of  which  have 
been  introduced. 

TOOLS  AND  APPLIANCES 

Generators'  tools  include  many  that 
are  familiar  to  the  plasterer,  while  for 
finishing  concrete  surfaces  he  uses  hacking 
tools  (see  p.  244),  the  idea  of  which  has  been 
borrowed  from  the  mason's  kit.  The  tamp- 
ing or  punning  irons,  for  ensuring  that  the 
concrete  is  well  tamped  around  the  reinforce- 
ment, may  be  of  the  shapes  shown  in  Figs. 
143  to  146  and  152.  Other  punning  tools, 
special  spades,  etc.,  are  shown  by  Figs.  147 
to  151. 


Fi«.  143  Fig.  144  Fig.  145  Fig.  146         Fig.  147.-Perforated 

Figs.   143  to  146. — Four  Patterns  of  Iron    Tamper  Spade 


The  matter  is  entirely  one  for  the  good 
judgment  of  the  foreman.  The  mixing 
board,  measuring  about  15  ft.  by  15  ft., 
should  be  constructed  of  good,  straight,  dry 
boards  of  even  thickness,  braced  together 
at  each  end,  and  framed  up  on  scantlings  of 
2  in.  by  4  in.,  the  longitudinal  ones  projecting 


For  cutting  bars,  a  hack  saw — preferably 
a  machine  saw — is  useful  for  large  sizes  ; 
bars  can  also  be  severed  by  nicking  and  then 
breaking.  Hand-operated  shearing  machines 
are  available  for  rods  of  ordinary  size. 
Small  bars  used  as  stirrups  can  be  cut 
through  with  a  hammer  and  chisel,  with  a 


IN  PILLARS  AND  BEAMS 

A         -   the  effective  area  of  the  pillar. 

Ac  -  area  of  compressional  reinforcement  (in 
sq.  in.). 

AE  =  area  equivalent  to  some  given  area  or  area 
of  an  equivalent  section  or  equivalent 
area. 

As-  -  cross-sectional  area  of  a  vertical  or 
diagonal  shear  member,  or  group  of 
shear  members,  in  the  length  p,  where 
p  =  pitch  of  stirrups. 

At        --  area  of  tensile  reinforcement  (in  sq.  in.). 

A7/  -  area,  of  vertical  or  longitudinal  reinforce- 
ment in  sq.  in. 

a  -  arm  of  the  resisting  moment  or  lever  arm 
(in  in.). 

a'        =  arm  ratio  =  a'd  .  • .  a'd  =  a. 

oc  =  depth  or  distance  of  the  centre  of  com- 
pression from  the  compressed  edge. 

B  M  =  bending  moment  of  the  external  loads  and 

reactions  (in  Ib.  in.). 
Generally,  6  =  breadth. 

IN  TEE  BEAMS 

6         —  breadth  of  flange  of  beam  (in  in.). 
br       =  breadth  of  rib  of  T  beam  (in  in.). 

IN  BEAMS 
c        =  compressive  stress  on  the  compressed  edge 

of  the  concrete  (in  Ib.  per  sq.  in.). 
C         =  total  compression  on  the  concrete  (in  Ib. 

per  sq.  in.). 

IN  PILLARS 

c  =  working  compressive  stress  on  the  con- 
crete of  the  hooped  core. 

cs  =  compressive  stress  in  the  steel  (in  Ib.  per 
sq.  in.). 

IN  CIRCULAR  SECTIONS 

Generally,  d  =  diameter. 

IN  RECTANGULAR  SECTIONS 
Generally,  d  =  depth. 

IN  PILLARS 
d        =  the  diameter  of  the  hooped  core  in  in. 

IN  BEAMS 

d        =  effective  depth  of  the  beam  (in  in.). 

dc        =  distance  from  compressed  edge  to  centre 

of  compressional  reinforcement. 
rf.s        -=  total  depth  of  the  slab  (in  in.). 


IN  PILLARS 

d1  =  distance  between  the  centres  of  vertical 
bars  measured  perpendicular  to  the 
neutral  axis. 

E,-  -  elastic  modulus  of  concrete  (in  Ib.  per  sq. 
in.). 

E,       =  elastic  modulus  of  steel  (in  Ib.  per  sq.  in.). 
I          -  inertia  moment  of  a  member. 
I          -  length  of  a  pillar  or  effective  length  of 
span  of  beam  or  slab. 

Es 
TO  -  modular  ratio  =  vv  • 

jfic 
M  S  ==  maximum  shear. 

n  =  neutral  axis  depth — i.e.  depth  of  neutral 
axis  from  the  extreme  compressed  edge 
(in  in.). 

n'        =  n/d  =  the  neutral  axis  ratio  .  • .  n'd  =  n- 

O        =  perimeter  or  circumference  of  bars. 

P        =  total  safe  pressure. 

IN  PILLARS 

p  =  the  pitch  of  the  laterals  in  in.  (i.e.  the 
axial  spacing  of  the  laterals). 

IN  SHEAR  FORMULAE 

p  =  pitch  or  distance  apart  (centre  to  centre) 
of  the  shear  members  or  groups  of  shear 
members  (measured  horizontally). 

IN  BEAMS 

r  =  A./bd  =  ratio  of  area  of  tensile  reinforce- 
ment to  the  area  bd. 

R1      =  left-hand  reaction. 

R2      =  right-hand  reaction. 

S        =  total  shear  in  Ib.  at  a  vertical  section. 

s  =  intensity  of  the  shearing  stress  on  con- 
crete in  Ib.  per  sq.  in. 

ss  =  shearing  stress  on  the  steel  (in  units  of 
force  per  unit  of  area). 

T       =  total  tension  in  the  steel  (in  Ib.). 

t         =  tensile  stress  on  the  steel  (in  Ib.  per  sq.  in. ) . 

IN  PILLARS 

V       =  volume  of  hooped  core  in  cub.  in. 

W      =  total    working    load    or    weight    on    any 

member. 

w        —  weight  or  load  per  unit  oi  length  of  span. 
y         =   eccentricity  of  the  load  measured  from  the 

centre  of  the  pillar  (in  in.). 
Z        =  section  modulus. 


In  Gordon's  formula: — R,;=  total  safe  resistance  to  compression. 

to  compression. 


total  resistance  of  material 


Note. — The  above  is  based  on,  and  modified  from,  the  notation  proposed  by  the  Concrete  Institute  and  adopted 
by  the  Royal  Institute  of  British  Architects,  but  certain  symbols  have  been  omitted,  since  they  are  not  used  in  the 
book,  and  a  few  others  have  been  added.  The  notation  is  built  up  on  the  principle  of  an  index,  the  significant 
word  in  each  term  having  been  abbreviated  to  the  initial  letter,  subscript  letters  being  added  in  many  cases. 
Capital  letters  indicate  moments,  areas,  volumes,  total  forces,  total  loads,  etc.  Small  letters  indicate  intensity 
of  forces,  intensity  of  loads,  intensity  of  stresses,  lineal  dimensions  (lengths,  distances),  etc. 


ERECTION    OF    A    BUILDING 


129 


hammer  and  anvil  block,  as  shown  in 
Fig.  153,  or  with  cutting  pincers  or  pliers, 
several  excellent  patterns  of  which  are  now 
available. 


bending,  especially  in  the  case  of  bars  partly 
embedded  in  concrete.  The  claw  wrench 
(Fig.  156)  is  helpful  for  bending  ends  of 
stirrups  over  bars,  and  is  believed  to  have 


Fig.    148.          Fig.   149.— Tamper  for  Producing 
-Ross  Spade  Fine  Surface 


Fig.   150.— Special 
Spade  for  Facing 


Fig.  151.— Andrews 
Tamper 


been  introduced  by  the  Coignet  firm.  The 
wrench  shown  by  Fig.  157  answers  a  similar 
purpose.  By  means  of  the  key  or  twister 
shown  by  Fig.  158,  the  ends  of  stirrups  can 
be  twisted  together.  For  the  wiring  together 
of  the  reinforcements  where  they  intersect 
one  another,  practical  workers  have  their 


Fig.   152.— Wooden  Tamper 

Bending  appliances  include  hand  tools 
and  machines.  The  wrenches  shown  by 
Figs.  154  and  155  are  useful  for  general 

9 


Fig.   153. — Cutting  Anvil  and  Hammer 

own  individual  methods  which  they  believe 
to  be  best,  but  there  are  undoubted  advan- 
tages in  employing  a  simple  tool  of  the  type 
introduced  in  the  United  States  under  the 
name  of  the  Curry  tyer.  The  Curry  tyer 
is  about  12  in.  long,  and  the  method  of 
using  it  is  shown  in  Fig  159.  The  wire  ties 
used  are  a  few  inches  long,  the  actual  length 
depending  upon  the  thickness  of  the  bars, 


130 


REINFORCED    CONCRETE 


Fig.  154 


Fig.  155 


Figs.    154    and     155.— 

Wrenches   for  Bending 

Ends  of  Bars 


Fig.  156 


Fig.  157 


Figs.   156   and    157.— 

Wrenches  for  Bending 

Ends  of  Stirrups 


Fig.   158. — Key  or  Twister 

for  Bending  Ends  of 

Stirrups 


Fig.  161. — Kennedy  Bar  Bending  Machine  No.  1 


Fig.   159.— Curry  Tyer 


Fig.   162. — Kennedy  Bar  Bending  Machine, 
Geared  Pattern 


a 

D 

6 

D 

D 

-C 

It 

1 

i 

A 

n~n 

--:-"---:::* 

_/i\_ 
B 

* 

"ttn  z  ' 

D 

1! 

^ 

! 

lf\T     

E        ''N;\ 
XC' 

-A 

/c 

..  ^  r 

B 

Ex         F 

--F--^"-"-^ 

f 

I  !( 

1                             S  \ 

1" 

^ 

,  ;• 

1  „     "    "      m       _ 

XA                          D 

u 

U 

L~^> 

Fig.   160. — Elevation  and  Plan  of  Bench  Bending  Machine 


ERECTION    OF    A    BUILDING 


and  they  have  a  loop  at  each  end.  The  tool 
itself  is  a  twisting  appliance  resembling  in 
principle  an  automatic  screwdriver,  there 
being  a  sliding  handle  working  up  and  down 
in  a  spiral  groove  machined  on  a  revolving 
shaft  inside  the  handle.  The  working  end 


of  the  tool  has  two  hooks  ;  over  one  of  these 
is  passed  one  loop  of  a  wire  tie,  the  tie  is 
then  passed  around  the  place  of  inter- 
section, and  the  other  loop  is  caught  over 
the  remaining  hook.  The  sliding  handle  is 
then  drawn  back  smartly,  with  the  result 


-_- 


Fig.    163. — Making  Bend  to   Given  Inside 
Measurement 


Fig.   164.— Making  Bend  to  Given  Outside 
Measurement 


Fig.    165.— Making  a  Double  Set 


Fig.   166.— Making  Sharp  Bend  in  Thin  Bar 


132 


REINFORCED    CONCRETE 


that  the  two  ends  of  the  .tie  are  twisted 
together  tightly.  Ties  can  be  made  by  this 
means  at  least  twice  as  fast  as  when  pliers 
and  straight  wire  are  used,  besides  which 
the  work  is  more  uniform. 

In  all  cases  it  is  advisable  to  bend  bars 
cold  ;  a  course  that  is  now  easily  possible 
with  bars  up  to  1|  in.  diameter.  The  small 
bars  (up  to  1  in.  in  diameter)  can  be  bent 
on  a  long,  strongly  constructed,  temporary 
bench  to  which  is  attached  a  vice  for  holding 
the  bar  firmly,  the  end  to  be  bent  lying 
between  two  steel  pins  fixed  in  a  template 
on  the  bench  ;  a  strong  steel  lever  with  a 
hole  in  it  to  fit  over  one  of  the  pins  must  be 
used,  the  bar  lying  across  the  lever  between 
the  steel  pin  on  which  the  lever  rotates  and 
the  pin  fixed  on  the  lever.  By  pulling  the 
lever  round,  the  bar  is  bent  to  any  angle. 
Fig.  160  shows  a  machine  of  this  type.  To 
obtain  the  correct  angle,  a  clear  chalk  line  is 
marked  on  the  bench  to  the  angle  required, 
and  the  bar  is  pulled  over  to  lie  on  it.  The 
machine  above  described  can  be  made  on 
any  job  at  a  cost  of  about  30s.,  and  possibly 
less,  as  the  contractor  would  probably  have 
sufficient  stock  material  at  his  workshops  to 
enable  him  to  fit  it  up.  The  bending  bench 
shown  by  Fig.  160  and  above  referred  to  is 
based  on  one  illustrated  in  the  Coignet  hand- 
book. The  rigid  bench  has  a  hard-wood  top  A, 
vice  B,  and  bar  rest  c.  The  lever  D  rotates 
on  a  fixed  pivot  E,  and  has  holes  for  pins  or 
rollers  F.  Such  a  machine  is  suitable  for 
bars  up  to  1 J  in.  diameter.  A  right  and  left 
double  bar  bender  on  somewhat  similar 
lines  is  shown  in  the  photographic  view 
(Fig.  167). 

Another  method  of  bending  bars  cold  is 
by  means  of  a  special  bending  machine,  and 
Kennedy's  may  be  mentioned  as  being  a 
most  effective  device  by  means  of  which  cold 
bars  up  to  1£  in.  in  diameter  may  be  bent. 
The  Kennedy  bar-bending  machine  is 
made  in  four  patterns.  No.  1  (see  Fig.  161) 
has  a  direct  lever,  and  bends  bars  up  to  |-in. 
diameter,  the  smallest  radius  being  \  in.  ; 
its  weight  is  25  Ib.  As  shown,  the  bar  is 
held  between  a  stop  and  a  central  mandrel 
or  former,  the  bending  force  being  applied 
by  a  lever  through  a  pulley  in  contact  with 
the  bar.  No.  2  (see  Fig.  162)  is  geared  and 
will  bend  1-in.  bars  to  a  £-in.  radius  ;  its 
weight  is  124  Ib.  No.  3  is  much  the  same  as 
No.  2,  but  in  addition  there  is  a  ratchet 
arrangement,  while  No.  4  is  a  powerful 
worm-geared  machine  for  bending  li-in. 


bars.  To  obtain  a  given  measurement  inside 
to  inside  of  bends  A  B  (Fig.  163),  having 
made  one  bend,  place  the  bar  as  shown,  mark 
off  distance  required,  mark  the  line  c  D 
square  with  A  B,  and  then  make  the  second 
bend.  When  a  given  measurement  E  F  (Fig. 
164)  is  to  be  taken  over  all,  make  the  first 
bend,  mark  off  desired  measurement  G 
along  the  bar,  square  off  from  G  as  before, 
taking  care  to  allow  for  thickness  of  the  bar, 
and  then  make  the  bend.  To  make  a  double 
set,  having  made  the  first  bend,  reverse 
the  bar  and  place  it  against  the  stop,  as  in 
Fig.  165  ;  let  a  straightedge  be  put  against 
the  mandrel  or  former,  parallel  with  A,  so 
as  to  indicate  the  extent  of  the  second  bend. 
For  making  a  sharp  bend  in  thin  bars,  an 
extra  former  A  (Fig.  166),  shaped  to  suit  the 
section  of  the  material,  is  inserted,  the  bar 
placed  between  A  and  the  central  former  B, 
and  the  lever  c,  carrying  a  stop  instead  of  a 
pulley,  is  pushed  round. 

When  large  diameter  bars  have  to  be  bent, 
the  aid  of  heat  has  to  be  sought,  but  they 
should  not  be  heated  above  a  cherry  red — 
that  is  to  say,  they  should  be  heated  only 
just  sufficient  for  the  purpose.  For  stirrups 
and  short  lengths  of  small  diameter  bars, 
such  as  T3F  in.  and  \  in.,  a  single  bend  can  be 
made  quickly  simply  by  bending  the  bar 
round  a  steel  pin  on  a  bench  by  means  of  a 
wrench,  as  shown  in  Fig.  168  ;  another  way 


Fig.   168.— Method  of  Bending  Small  Bars 

is  to  fix  a  piece  of  steel  tubing  to  a  vertical 
wooden  post,  nail  a  stop  underneath  it,  place 
the  bar  in  the  tube,  and  pull  it  over,  by 
which  means  stirrups  can  be  made  very 
quickly. 

In  almost  every  case,  the  bars  in  a  slab  or 
wall  have  their  ends  just  bent  over  for  the 
purpose  of  forming  a  key  or  grip  in  the  con- 
crete ;  such  bends  can  be  made  with  a  blow 
or  two  from  a  hammer. 

DRIVEN   PILES 

Now  that  the  contractor  has  everything  on 
the  site  for  the  erection  of  his  building,  the 


ERECTION    OF    A    BUILDING 


pile  making  and  driving  is  the  first  tiling  to 
which,  he  must  turn  his  attention. 

The  Pile  Skeleton. — For  the  steel  skele- 
ton, the  correct  bars  specified  must  be 
selected  and  placed  on  trestles  close  enough 
to  prevent  sagging ;  the  bars  are  then 
placed  in  wood  templates,  which  may 
be  square  or  circular  according  to  the 
engineer's  requirements.  These  templates 
must  be  removed  as  soon  as  sufficient  bind- 
ing is  done  to  hold  the  longitudinal  bars  in 
their  correct  positions.  The  next  process 
is  to  add  the  binding,  which  may  be  done 
in  several  different  manners  according  to 
the  design  of  the  pile.  In  the  case  of  a 
square  pile  of  the  Hennebique  type,  links 
have  to  be  made  which  wrap  round  two  rods 
at  a  time,  as  clearly  illustrated  in  Pig.  169. 
These  links  must  be  exactly  spaced  at  the 
given  pitch  shown  in  the  illustration.  For  a 
circular  pile  of  the  Coignet  type  (shown 
by  Fig.  170)  the  links  or  ties  encircle  the 
bars,  and  the  ends  of  the  links  are  bent 
tightly  round  a  single  bar,  and  at  the  point 
where  the  links  touch  the  bars  they  should 
be  tightly  tied  with  annealed  wire.  For  a 
pile  of  the  Considere  type,  which  is  octagonal 
in  shape,  the  binding  is  of  spiral  form,  the 
spiral  being  first  wound  on  a  drum  and  then 
threaded  on  to  the  bars.  This  type  of  pile 
is  shown  in  Fig.  171.  The  pitch  of  the  spiral 
must  be  strictly  adhered  to.  Whilst  these 
skeleton  frameworks  are  being  made,  the 
joiners  will  be  preparing  the  wooden  moulds 
for  their  reception,  a  detailed  description 
of  which  is  given  in  the  next  chapter.  As 
the  moulds  can  be  used  and  re-used,  it  is 
at  the  foreman's  discretion  how  many  he 
makes,  but  there  should  be  sufficient  to 
allow  of  the  work  continuing  without 
hindrance  ;  the  more  times  one  mould  can 
be  employed  the  greater  the  economy,  and 
it  will  here  be  useful  to  state  that  in  ordinary 
circumstances  pile  moulds  can  be  removed 
in  four  or  five  days  after  the  concreting. 
When  several  moulds  are  completed,  the 
steel  skeletons,  which  have  been  carefully 
lifted  from  the  trestles  and  laid  on  planks 
on  level  ground,  should  be  taken  up  and 
placed  in  the  moulds,  and  the  cast-iron  pile 
shoe  will  then  have  to  be  fixed  to  the  bottom 
of  the  steel  framework  in  the  mould.  Having 
made  a  careful  inspection  to  see  that  the 
steel  framework  is  lying  true  in  the  mould, 
a  gang  of  concreters  with  punning  rods  or 
rammers  should  be  set  to  work  ;  every  bit 
of  concrete  placed  in  the  mould  should  be 


carefully  punned  so  that  the  steel  is  com- 
pletely covered.  Keen  supervision  should 
be  exercised,  because  if  by  any  chance  any 
cavities  form,  or  there  is  a  large  piece  of 
aggregate  loose  in  a  pile,  disaster  will  over- 
take that  pile  under  the  first  blow  of  the 
driving-ram. 

In  very  important  work  it  is  better  to 
cast  the  piles  in  a  vertical  position,  this 
being  considered  to  give  a  more  uniform 
strength  over  the  cross  section. 

When  the  concreting  is  completed,  the 
uppermost  face  must  be  carefully  levelled 


Fig.  169.— 
Hennebique 
Square  Pile 


Fig.   170.— 

Coignet 
Round  Pile 


Fig.   171.— 

Considere 

Octagonal  Pile 


off,  and  the  pile  may  be  left  to  dry.  In  hot 
weather  the  concrete  should  be  watered  (by 
means  of  an  ordinary  watering-pot  fitted 
with  a  rose)  daily  for  a  week  or  ten  days  ; 
but  this  should  not  be  done  until  the  con- 
creting has  been  completed  for  ten  or 
twelve  hours.  Ordinarily,  a  well-made  pile 
can  be  safely  driven  in  six  to  seven  weeks 
after  making.  In  cold  weather  the  newly- 
made  pile  must  be  carefully  protected  from 
frost  by  covering  with  sacking,  etc.  When 
the  piles  have  been  in  the  moulds  for 
four  or  five  days,  as  already  stated,  the 
moulds  may  be  struck — work  that  must  be 
carefully  done,  so  that  the  arrises  are  not 
chipped  off  and  the  steel  bared.  This 


Fig.   172. — Lidgerwood  Pile-driving  Engine  and  Reinforced    Concrete  Pile 


134 


ERECTION    OF    A    BUILDING 


applies  to  circular  as  well  as  square  and 
other  piles,  because  the  "  circular  "  pile  has 
two  flat  sides  about  4  in.  wide  for  the  pur- 
pose of  facilitating  the  driving,  an  arris 
being  therefore  formed  at  the  juncture  of 
the  flat  side  and  the  circular  side,  as  shown 
in  Fig.  170.  In  cases  where  a  wooden  dolly 
alone  is  used  without  a  cast-iron  helmet 
on  the  head  of  a  pile  during  driving,  it  is 
advisable  when  making  the  pile  to  use  a 
richer  mixture  for  a  distance  of  from  2  ft. 
to  2  ft.  6  in.  down  from  the  head,  the  object 
being  to  strengthen  the  head  so  that  it  will 
not  be  seriously  damaged  under  the  impact 
(a  wooden  "  dolly  "  actually  takes  the  blow 
of  the  ram  or  monkey).  The  proportions 
of  the  concrete  for  use  in  piles  need  to  be 
determined  scientifically. 

Driving  of  Piles. — Eeinforced  concrete 
piles  are  driven  with  a  special  pile-driving 
machine  or  engine,  among  the  best  known 
being  the  Lecour,  Sykes,  and  Lidgerwood. 
The  last-named  is  shown  in  Fig.  172.  The 
drivers  are  generally  fitted  with  a  two-ton 
ram,  which  is  hoisted  by  means  of  a  steam 
winch.  The  ram  can  be  driven  at  speeds  up 
to  thirty  or  forty  blows  a  minute.  Care  and 
judgment  must  be  exercised  to  drive  with 
steady,  uniform  blows  with  a  short  drop, 
otherwise  damage  will  be  done  by  splintering 
and  cracking  the  pile-head,  however  good 
and  hard  the  concrete  may  be.  With  a 
trustworthy  ganger  driver  who  will  take 
care  piles  can  be  driven  without  a  crack. 
Owing  to  the  great  weight  of  a  long  pile, 
say  one  from  30  ft.  to  50  ft.  in  length,  special 
arrangements  should  be  made  for  careful 
handling,  hoisting,  and  placing  in  position 
for  driving.  When  a  pile  is  hoisted  into 
its  correct  position,  it  must  be  plumbed  up, 
bolted  on — or  rather  through — the  two 
leaders  of  the  piling  frame,  and  then  plumbed 
up  again  when  lowered  before  any  blows 
are  given.  When  a  cast-iron  helmet  is 
used,  as  generally  is  the  case  with  a  Henne- 
bique  pile,  a  strong  cushion  of  sawdust  in 
sackcloth  is  packed  inside  the  helmet  to 
relieve  the  head  of  the  pile  from  the  severe 
impact  caused  by  the  fall  of  the  ram,  thus 
saving  the  head  of  the  pile  from  shattering. 
When  a  helmet  is  not  used,  an  elm  block  or 
dolly  about  3  ft.  long  is  placed  on  the  head 
of  the  pile  for  the  same  purpose  as  the 
helmet.  However,  Considere  piles,  by  reason 
of  the  special  form  of  their  reinforcing,  are 
generally  driven  direct,  without  any  helmet 
or  dolly. 


Everything  being  ready,  the  pile  driver 
proceeds  with  the  driving,  which  is  con- 
tinued until  the  specified  set  or  stopping 
place  is  reached  ;  the  set  is  specified  in  the 
engineer's  or  architect's  specification,  and 
must  always  be  strictly  observed.  The  test 
required  for  the  set  of  a  pile  usually  is,  that 
it  does  not  sink  more  than  J  in.  or  J  in. 
under  ten  blows  of  a  two-ton  ram  having 
a  drop  of  3  ft.  ;  but  much  depends  on  the 
soil  and  the  nature  of  the  strata  through 
which  the  pile  has  to  penetrate. 

It  is  desirable  to  note  a  few  precautions 
that  should  be  taken  by  the  clerk  of  works 
in  testing  a  pile  set.  The  general  practice 
is  first  to  determine  the  distance  of  the  drop 
of  the  ram  to  the  head  of  the  pile  ;  this  is 
done  by  clearly  marking  on  the  piling 
frame  by  means  of  a  chalk  line  the  height 
to  which  the  ram  is  to  be  hoisted  for  each 
blow.  For  marking  the  pile,  a  gauge  rod 
is  hung  from  the  leaders  of  the  piling  frame 
in  such  a  manner  that  it  may  swing.  A 
line  is  drawn  on  the  pile  at  the  level  reached 
by  the  bottom  of  the  gauge.  The  specified 
number  of  blows  is  then  given,  and  the  pile 
is  again  marked  with  a  pencil  at  the  bottom 
of  the  gauge  rod.  The  space  made  between 
the  line  drawn  on  the  pile  before  the  delivery 
of  the  blows  and  the  line  drawn  afterwards 
determines  the  going  or  sinking  of  the  pile 
under  the  blows. 

In  witnessing  a  test,  the  clerk  of  works  or 
inspector  should  observe  the  following  points 
very  minutely : — (1)  When  a  helmet  is  used 
for  capping  a  pile,  see  that  the  sawdust 
cushion  packed  inside  is  the  same  one  that 
has  been  used  during  the  driving  of  the  pile, 
and  that  another  has  not  been  substituted, 
or  that  a  new  one  has  not  been  packed  into 
the  helmet  and  the  old  one  placed  under  it 
so  that  on  looking  up  the  inspector  sees  the 
old  cushion  apparently  undisturbed.  Should 
a  new  cushion  be  inserted,  the  pile  does  not 
get  the  full  benefit  of  the  blow,  and  conse- 
quently the  test  is  not  accurate. 

(2)  See  that  the  ram  or  monkey  is  raised 
during  the  test  blows  to  the  full  height 
marked  on  the  piling  frame,  and  see  that 
the  men  holding  the  guy  ropes  do  not  regu- 
late the  fall  of  the  ram  so  that  it  loses  its 
force  just  before  striking  the  head  of  the 
pile.  The  precaution  should  be  taken  of 
checking  the  length  of  the  gauge  rule  used 
for  measuring  the  fall  of  the  ram  with  an 
ordinary  rule.  It  is  as  well  to  measure  the 
gauge  rod  every  time  before  it  is  used. 


136 


REINFORCED    CONCRETE 


(3)  Watch  closely  the  angle  at  which  the 
pencil  is  held  in  marking  the  lines  on  the 
pile,  as  a  somewhat  cute  device  which  needs 
to  be  guarded  against  has  come  frequently 
under  our  observation.     The  pencil  was  held 
at  an  acute  angle  from  the  bottom  of  the 
swing  gauge,  thus  marking  the  pile  |  in.  or 
even  J  in.  above  what  it  would  be  if  the 
pencil  were  held  at  right  angles  to  it ;  then, 
after  driving,  the  second  line  was  drawn 
with  the  pencil  at  right  angles  to  the  gauge. 
Obviously,  the  space  between  the  two  lines 
was  less  than  it  would  be  if  both  of  the  lines 
had  been  drawn  with  the  pencil  at  right 
angles  to  the  bottom  of  the  gauge.    An  effec- 
tive cure  is  for  the  clerk  of  works  to  insist 
on  marking  the  pile  himself. 

(4)  When  pile  driving  is  carried  out  at 
night,  see  that  the  pile  shoes  are  not  knocked 
off  before  lowering  the  pile  into  position  for 
driving. 

Reinforced  concrete  piles  are  generally 
placed  in  groups  of  two,  four,  or  six.  It 
may  happen  that  in  driving  the  first  pile 
of  the  group  a  set  cannot  be  obtained,  and 
the  pile  is  driven  right  into  the  ground,  even 
then  failing  to  pass  the  test,  always  going, 
say,  y1^  in.  to  \  in.  more  than  is  permissible. 
Where  such  an  event  is  likely,  the  driving 
of  the  first  pile  should  be  stopped,  and  the 
second  one  started.  The  next  day,  the  first 
pile  should  be  given  about  120  blows  with 
a  3-ft.  drop  of  the  ram  before  applying  the 
test,  when,  in  the  majority  of  cases,  the  pile 
will  not  go  more  than  -^  in.  and  probably 
not  as  much  as  that.  Should,  however,  the 
pile  still  continue  to  go,  it  must  be  spliced 
or  added  to  in  some  other  way.  The  splice 
must  be  designed  by  the  engineer  ;  as  a 
rule,  extra  rods  are  placed  at  the  joint  of 
the  old  pile  and  the  addition.  The  concrete 
of  the  pile  must  be  cut  away  down  for  some 
distance  so  that  all  the  reinforcement  is 
bared.  The  new  steel  for  the  additional 
length  is  properly  framed  up  and  joined  to 
the  projecting  reinforcement  of  the  pile. 
A  wooden  mould  is  placed  round  the  steel 
framework  to  the  height  required  for  the 
additional  length,  and  the  whole  is  now 
ready  to  be  rilled  with  concrete  exactly  as 
for  a  column  (described  later).  When  the 
concrete  is  sufficiently  dry,  the  mould  is 
taken  off,  and  five  or  six  weeks  later  the 
driving  may  be  re-started. 

Cast  -  in  -  place  Piles. — Concrete  piles 
cast  in  place  are  now  frequently  used  in 
foundation  work,  one  of  the  best  known  of 


the  type  being  the  "  Simplex "  (Fig.  173), 
in  which  system  a  hollow  cylinder  is  driven 
to  a  bearing,  such  as  ballast,  and  then 
gradually  withdrawn,  the  hole  left  by  the 
cylinder  being  filled  in  with  concrete  and 
heavily  rammed.  This  cylinder  is  made  of 
about  ^-in.  lap-welded  steel  about  14  in.  to 
16  in.  in  diameter  and  from  30  ft.  to  40  ft. 
long,  made  up  in  two  or  three  sections  cross- 
welded.  The  upper  portion 
is  strengthened  by  a  |-in.  riveted 
band  about  18  in.  deep  and  a 
narrower  band  \  in.  thick  riveted 
to  the  bottom  end  just  above 
the  shoe.  The  pile  is  provided 
with  a  special  alligator  jaw  or 
shoe,  which  is  closed  during  the 
driving  to  prevent  the  entrance 
of  the  surrounding  materials 
into  the  cylinder  or  pipe.  The 
jaw,  which  is  securely  attached 
to  the  pipe  by  cable  hinges,  and 
can  be  used  over  and  over  again, 
is  composed  of  two  symmetrical 
wedge  parts,  which  are  kept 
closed  during  the  driving  by  the 
earth  pressure.  In  hard  soils  a 
detachable  cast-iron  point  is 
often  used,  it  being  left  in  per- 
manently. The  cylinder  or  form 
is  driven  by  a  pile  driver,  which 
is  very  little  different  from  the 
ordinary  type,  except  that  it  is 
fitted  with  a  strong  pulling  de- 
vice attached  to  the  leaders  for 
the  purpose  of  withdrawing  the 
form  after  it  has  been  driven. 
When  the  cylinder  has  been 
driven  to  refusal,  the  driving 
head  is  hooked  to  the  hammer 
and  wire  attachment,  and  both 


s 


Fig.  173.- 

Simplex 

Pile 


are  hauled  to  the  top  of  the  leaders  during 
the  concreting.  A  batch  of  concrete  of  about 
5  cub.  ft.  is  dropped  into  the  form,  after 
which  the  entire  form  is  raised  about  1  ft. 
by  a  pulling  device.  The  concrete  is  then 
rammed  by  dropping  a  heavy  rammer  into 
the  mass.  The  impact  opens  the  jaws  and 
forces  the  concrete  out  into  the  space  made 
by  withdrawing  the  form.  The  process  is 
continued  until  the  whole  form  can  be  with- 
drawn, and  the  space  it  occupied  is  filled 
with  concrete,  which  sets  and  hardens,  and 
so  becomes  a  concrete  pile.  This  pile  is 
sometimes  reinforced  by  bars  being  inserted 
and  so  arranged  as  to  allow  the  rammer  for 
the  concrete  to  be  dropped  down.  By  the 


ERECTION    OF    A    BUILDING 


use  of  a  wet  and  sloppy  concrete  the  rammer 
need  not  be  used,  as  the  concrete  will  settle 
down  and  surround  every  part  of  the  rein- 
forcement. The  rods  inserted  should  pro- 
ject through  the  head  of  the  pile  for  the 
purpose  of  joining  up  the  beams  at  their 
junction  at  the  pile  head. 

"  COMPRESSOL  "  CONCRETE  PILES 

The  "  Compressol "  system  is  the  in- 
vention of  a  French  engineer,  S.  L.  Dulac,  and 
its  name  indicates  the  compressing  of  the 
soil.  The  plant  includes  three  rams  which 
are  of  different  shapes  and  sizes,  as  shown 
in  Fig.  174.  The  "  borer  "  is  of  a  long  conical 


Fig.  174. — "Compressol" 
&OE1E.R.  Borer,   Rammer,     and 

Tester 

shape,  with  a  sharp  point,  the  latter  having 
a  special  cavity  which  brings  back  a  sample 
of  the  soil  reached  at  each  blow,  so  enabling 
an  opinion  to  be  formed  as  to  the  necessity 
of  carrying  the  foundation  to  a  greater  depth. 
The  diameter  of  this  ram  at  the  upper  part 
is  2  ft.  4  in.  and  the  length  about  6  ft., 
whilst  the  weight  is  32  cwt.  The  second 
ram  is  called  the  "  rammer,"  which  has  an 
ogival  shape  with  a  length  of  3  ft.  and  a 
diameter  of  2  ft.  2  in.,  the  weight  being 
30  cwt.  The  third  ram  is  the  "  tester,"  and 
weighs  exactly  1  ton ;  unlike  the  others,  it 
is  made  to  fall  with  the  large  base  downwards. 
It  is  shaped  somewhat  like  a  frustrum,  and 
the  greater  diameter  is  2  ft.  8  in.  These 
rams  are  worked  with  a  frame,  in  a  similar 
manner  to  that  employed  for  dropping 
the  monkey  in  piling  work,  with  a  steam 
winch  and  boiler  ;  the  frame  is,  in  addition, 
supported  on  a  revolving  base  plate  which, 
in  turn,  rests  on  a  set  of  wheels  in  order  to 
allow  the  frame  to  be  moved  longitudinally, 
as  well  as  about  its  own  centre.  There  are 
practically  two  systems,  one  of  which  is 
used  for  deep  foundations  and  the  other  for 
merely  a  surface  compression!  of  the  soil 
when  the  latter  is  of  a  poor  nature. 


In  forming  the  deep  foundations  the 
"  borer  "  is  first  used,  and  this  is  drawn  up 
the-  frame  and  allowed  to  drop  on  to  the 
soil,  the  height  varying  according  to  the 
circumstances,  sometimes  reaching  as  much 
as  35  ft.  Fig.  175  shows  the  frame  and  the 
"  borer  "  in  position  during  the  execution 
of  the  work.  An  automatic  self-acting 
grip,  supported  by  a  pulley  and  chain,  is 
employed,  and  each  ram  is  provided  with  a 
specially  designed  rod  at  the  head,  as  shown 
in  Fig.  174,  which  allows  the  automatic 
grip  to  come  into  action.  As  soon  as  the 
ram  is  dropped,  the  automatic  grip  is 
lowered  to  pick  up  the  ram,  and  upon  the 
latter  being  wound  up  to  any  desired  level 
the  grip  comes  into  contact  with  a  special 
ring,  which  is  fixed  to  the  frame,  and  this 
automatically  releases  the  ram  and  allows 
it  to  fall ;  then  the  process  is  repeated. 
A  sufficient  number  of  drops  is  given  to 
make  a  circular  hole  in  the  soil ;  and  in  the 
operation  all  the  strata  adjoining  are  com- 
pressed to  such  an  extent  that  the  sides  of 
the  hole  hold  up  very  well  in  most  cases. 

The  process  of  boring  and  compressing  is 
continued  until  the  level  is  reached  at 
which  the  foundations  are  to  start,  and  then 
the  "  borer  "  is  withdrawn  and  several  large 
stones  are  thrown  into  the  circular  shaft 
and  subjected  to  severe  blows  with  the 
"  rammer,"  which  forces  them  outwards 
and  spreads  the  bottom  of  the  hole  until  it 
forms  a  wide  base,  in  some  cases  with  a 
diameter  of  6  ft.  or  7  ft.,  thus  giving  a  good 
footing  for  the  concrete  pier,  which  is  to 
come  above. 

When  the  base  is  properly  prepared  the 
process  of  filling  in  the  hole  with  concrete  is 
proceeded  with,  and  this  is  accomplished  by 
depositing  layers  about  16  in.  to  20  in.  thick, 
which  are  heavily  rammed  with  a  few  blows 
of  the  rammer ;  cement  concrete  is  used 
where  great  strength  is  required.  The 
effect  of  the  ramming  is  to  consolidate  the 
concrete  and  give  it  increased  strength ;  at 
the  same  time  the  surrounding  soil  is  much 
compressed  by  the  spreading  out  of  the 
concrete,  which,  when  completed,  has  been 
found  to  measure  6  ft.  and  more  in  diameter, 
as  against  the  original  size  of  the  hole,  which 
was  about  3  ft. 

Fig.  176  shows  the  finished  concrete  pier, 
and  indicates  clearly  that  the  object  of  the 
ramming  is  admirably  obtained.  The  photo- 
graph shows  how  the  concrete  is  forced  out 
to  a  greater  extent  when  passing  through  a 


138 


REINFORCED    CONCRETE 


soft  stratum,  and  thereby  effecting  a  greater 
compression  at  this  point,  making  a  more 
uniform  resistance  at  each  portion  of  the 
pier  and  giving  projections  which  materially 
assist  in  resisting  any  downward  pressure. 

It  is  usual,  when  constructing  these  piers 
in  actual  building  work,  to  use  large  stones 
at  the  base  of  the  pier  and  smaller  material 
when  nearing  the  top. 


when  the  ram  meets  the  pier  ;  that  is  to  say, 
30  tons  for  a  set  of  1  ft.  If  the  set  is  only 
|  in.  per  blow  the  bearing  power  will  obviously 
be  24  times  as  great,  or,  in  other  words, 
720  tons.  Against  this,  of  course,  must  be 
placed  a  factor  of  safety  to  allow  for  the 
loss  of  energy  due  to  vibration,  the  resist- 
ance of  the  air,  and  other  causes,  when 
testing. 


Fig.  175. — "  Compressol "  Frame  and  Borer  in  Ute 


Fig.   176. — "  Compressol ''  Pile 


The  bearing  power  of  each  pier  is  ascer^ 
tained  by  means  of  the  "tester."  When 
the  pier  is  nearing  completion,  the  rammer 
is  withdrawn  and  the  tester  is  substituted, 
this  being  used  to  give  a  volley  of  heavy 
blows  on  the  head  of  the  pier,  during  which 
time  the  set  is  ascertained.  Naturally,  the 
less  the  pier  is  sunk  by  each  blow  the  greater 
the  resistance  it  will  offer  when  carrying  a 
superincumbent  load.  The  weight  of  the 
ram  being  exactly  1  ton  and  the  fall  being, 
say,  30  ft.,  a  force  of  30  ton-ft.  is  obtained 


When  much  water  is  encountered  during 
the  boring  it  is  necessary  to  adopt  measures 
to  prevent  it  from  draining  into  the  well, 
and  this  is  accomplished  in  a  simple  manner. 
The  hole  is  rilled  with  ordinary  clay  up  to 
about  8  in.  above  the  water  level,  and  the 
boring  operation  started  afresh.  The  clay 
is  forced  outwards  and  compressed  around 
the  well,  forming  a  kind  of  watertight  lining, 
which  is  usually  sufficient  to  prevent  any 
great  inflow  of  water.  It  may  become 
necessary  in  some  cases  to  repeat  the  opera- 


ERECTION    OF    A    BUILDING 


139 


tion  two  or  three  times,  but  eventually  a 
perfectly  watertight  well  can  be  obtained. 

In  the  case  of  surface  compression  only 
(which  is  used  where  the  loads  are  light,  but 
where  the  soil  is  "  made  "  ground,  or  of  a 
similar  description  and  needs  improvement) 
the  "  borer  "  is  not  employed,  but  trenches 
are  made  about  3  ft.  deep  and  the  rammer  is 
used  to  make  a  hole  4  ft.  or  5  ft.  deep, 
which  is  then  filled  with  dry  stones,  bricks, 
or  similar  material  for  about  one-third  of 
the  depth,  these  being  subjected  to  a  few 
heavy  blows  with  the  rammer,  which  forces 
them  outwards  and  downwards  all  round 
into  the  soil.  This  process  is  repeated  two 
or  three  times  and  the  tester  is  then  used  to 
give  a  few  blows,  after  which  a  great  improve- 
ment can  be  seen  in  the  soil. 

This  method  is  employed  in  different  por- 
tions of  the  trenches  at  such  distances  apart 
as  may  be  necessary  under  the  particular 
conditions  of  the  case,  which  are  governed 
by  the  nature  of  the  soil  and  the  loads  to  be 
carried.  This  is  a  very  cheap  and  effective 
way  of  improving  the  foundations  for  a  light 
structure. 

Pile  Gaps. — When  all  the  piles  have 
been  driven  and  passed,  the  constructor  can 
proceed  to  cap  them,  and  to  form  the  con- 
necting beams  that  will  support  the  bottom 
floor.  Should  the  site  at  the  surface  be  of 
fairly  solid  substance,  no  bottom  planking 
will  be  necessary  for  the  pile  caps  and  beams, 
or  even  for  the  floor  slabs.  Fig.  177  is  a 
plan  of  a  typical  pile  cap. 

For  the  purpose  of  constructing  a  concrete 
cap  to  a  pile  that  has  been  driven  a  mould 
in  the  form  of  a  box  is  made  to  surround 


forcements  are  placed  in  the  beam  moulds, 
all  the  bars  properly  connected  up  at  their 
intersections  over  the  piles  (see  Fig.  178), 
and  the  concreting  begun.  While  the  con- 
creting is  being  done,  the  spaces  between 
the  beams  may  be  filled  in  to  the  level 


PILE  CAP 
2  P/iCS 


Fig.   177.— Plan  of  Typical  Pile  Caps 
and  Connecting  Beams 

of  the  under-side  of  the  slab,  by  which 
time  it  is  probable  that  the  moulds  to  the 
pile  caps  and  beams  may  be  struck,  taken 
away,  and  cleaned  for  re-use.  The  filling  in 
between  the  beams  up  to  the  level  of  the 
bottom  of  the  floor  slab  should  be  completed 
by  thoroughly  punning  and  levelling  off 
with  some  good,  small,  hard  material  such 
as  a  mixture  of  cinders  and  gravel,  clinker, 
etc.,  so  as  to  form  a  good  bottom  on  which 
to  lay  the  reinforcement  for  the  floor  slab 
(see  Fig.  178). 

THE    FOUNDATION    SLABS 

The  placing  of  the  slab  reinforcement  is 


NOTC  -  PllE  OAgJ  sm.'t>KO  AfJD 
TAKiU  IHTO   CAP 


PILES 


Fig.    178.  —  Section  showing  Pile  Caps,   Filling,   Beam  and  Slab 


each  group  of  piles.  The  reinforcement  has 
to  be  placed  in  each  of  the  boxes,  according 
to  the  engineer's  designs.  The  pile  caps  are 
connected  together  by  beams  which  are 
moulded  in  wooden  forms ;  the  beam  rein- 


the  next  proceeding.  The  reinforcement 
consists  of  bars  laid  to  form  a  meshwork,  or 
of  a  steel  mesh  formed  by  welding  or  "  weav- 
ing "  fine  rods,  or  by  "  expanding  "  steel 
plate.  Let  it  be  assumed  that  ordinary 


140 


REINFORCED    CONCRETE 


round  bars  are  to  be  used,  spaced  at  certain 
distances,  and  crossed  by  distributing  bars 
wired  at  the  intersections.  A  practical 
method  of  placing  the  main  bars  in  their 
correct  positions  is  to  obtain  two  straight 
pieces  of  boarding  about  4  in.  wide,  and  in 
this  to  cut  notches  for  the  bars  to  agree  with 
the  spacing  specified.  The  boards  are 
placed,  notches  upwards,  one  near  each 
beam,  and  the  bars  laid  to  fit  the  notches 


on  the  projecting  reinforcement,  which  con- 
sequently is  bent  anyhow. 

Concreting  the  Slab. — Concreting  the 
slab  reinforcement  already  laid  is  the  next 
process,  and  arrangements  should  be  made, 
if  possible,  to  carry  out  the  concreting 
over  one  complete  area  at  a  time,  so  as  to 
ensure  the  work  being  monolithic.  There 
are  two  good  practical  ways  of  doing  the 
work  so  as  to  ensure  that  the  reinforce- 


Fig.   179. — Foundation  Slab  Reinforcement  held  in  Notched  Templates 


(see  Fig.  179).  By  this  means  the  bars  will 
not  get  displaced.  The  notched  templates 
can  be  removed  shortly  after  the  concreting 
has  begun,  quite  a  small  quantity  of  concrete 
sufficing  to  hold  the  rods  in  place.  However, 
before  any  concreting  is  done,  the  cross  bars 
or  distributing  bars  have  to  be  laid  and 
wired  to  the  longitudinal  bars  at  every  (or, 
in  most  cases,  every  alternate)  intersection. 
The  work  is  not  difficult  or  complicated,  but 
it  requires  great  care,  and  certain  precautions 
need  to  be  observed.  For  example,  do  not 
allow  men  to  run  indiscriminately  over  the 
steelwork  after  it  has  been  laid  down ; 
instead,  form  runways  with  planks  packed 
up  from  the  ground  so  that  they  clear  both 
the  meshwork  and  the  beams,  it  being  borne 
in  mind  that,  at  this  period  of  the  work,  the 


Fig.    180. — Raft  Foundation  Beam  Reinforcement  on 
Wooden  Supports 


upper  or  compression  reinforcement  and  the 
stirrups  of  the  beams  are  projecting  above 
the  level  of  the  concrete  in  the  beams  ;  fore- 
men frequently  overlook  this,  and  allow 
boards  used  as  runways  to  rest  by  their  ends 


ment  does  not  come  out  on  the  surface. 
One  is  to  place  a  round  rod,  about  f  in. 
in  diameter,  under  the  meshwork,  and 
roll  it  forward  as  the  concreting  pro- 
ceeds, this  method  being  best  suited  for 
use  on  a  suspended  floor  with  flat  wood 
sheeting.  The  method  preferred  for  a 
foundation  slab,  as  in  the  present  case,  is 
to  place  a  layer  of  concrete  (say  about  1  in. 
thick)  from  each  barrow  load,  and  to  lift 
the  meshwork  through  it  by  means  of  a 
lifting  hook  (any  piece  of  steel  rod  hooked 
at  the  end).  The  hook  is  caught  round  one 
of  the  main  bars,  and  lifted  about  3  in.,  the 
mesh  being  given  a  slight  shake  and  then 
lowered  gently  to  rest  on  the  thin  concrete 
layer.  Then  the  remainder  of  the  concrete 
may  be  added,  well  tamping  all  the  time, 
but  not  too  heavily,  or  the  cement  will  work 
to  the  top.  Punning  or  tamping  needs  to 
be  done  lightly  but  thoroughly.  The  usual 
finish  is  to  level  the  slab  with  a  straightedge 
pressed  down  on  two  screeds,  and  worked 
with  a  short  motion. 

In  concreting  a  floor,  the 
concrete  should  be  carried 
forward  on  a  straight  line 
across  the  whole  of  the  width 
of  a  bay. 

In  the  concreting  of  the 
slab  at  present  being  considered,  those  places 
where  the  columns  (over  the  piles)  and  the 
walls  (resting  on  beams)  will  come  should 
not  be  smoothed  over,  but  left  quite  rough, 
so  that  good  joints  can  be  made  later. 


- 

u 


141 


142 


REINFORCED    CONCRETE 


The  foundation  for  the  main  building  is 
now  complete. 

The  raft  foundation  slab  for  the  engine 
house  is  constructed  in  a  different  manner 
from  the  foundation  for  the  main  building. 


before  doing  this  a  strong  grout  should  be 
poured  into  the  mould  to  strengthen  the 
key  with  the  slab  concrete.  To  complete  the 
work,  the  mould  must  be  filled  and  the  con- 
crete carefully  punned  so  that  every  particle 


Fig.    182. — Completed  Raft,   Kingsway  Church,   London 


The  preliminary  step  is  to  level  the  site 
either  by  excavating  or  by  making  up  with 
some  good  clean  material,  such  as  clinker, 
gravel,  and  ashes.  The  positions  of  the 
beams  are  next  carefully  set  out,  and  the 
steel  skeletons  for  the  beams  made  in  a  way 
similar  to  that  described  on  p.  133  for 
making  a  pile  skeleton.  The  skeletons  are 
placed  in  their  correct  positions,  and  set  to 
their  proper  and  exact  levels  on  temporary 
wooden  supports,  as  shown  in  Fig.  180.  The 
slab  reinforcement  should  then  be  laid  in  the 
same  manner  as  for  the  floor  just  previously 
described,  but  in  this  case  the  concreting  to 
the  slab  must  be  done 
first  instead  of  to  the 
beams,  the  concrete 
being  taken  up  to  the 
side  of  the  beam  frame- 
work, so  that  when  it 
set  sufficiently  to 


of  the  beam  reinforcement  is  properly 
covered.  The  moulds  may  be  stuck  four  or 
five  days  afterwards. 

Figs.  181  and  182  show  the  raft  for  Holy 
Trinity  Church,  Kingsway,  London,  which, 
on  the  completion  of  the  church  tower,  will 
carry  a  total  weight  of  2,500  tons,  and  which 
is  independent  of  the  foundation  of  the  rest 
of  the  building.  A  lower  slab,  12  in.  thick 
and  stiffened  by  small  cross-beams  each 
2  ft.  wide  by  2  ft.  9  in.  deep,  carries  two 
main  beams  each  4  ft.  wide  by  4  ft.  3  in. 
deep,  and  the  reinforcement  is  by  means  of 
patent  indented  bars. 


s 


Fig.    183. — Plan  of  Retaining  Wall  with  Tapering  Counterforts 


bear  the  weight  of  men 
and  materials  the  wooden  moulds  for 
the  beams  may  be  erected.  As  soon  as  the 
concreting  is  set,  the  wooden  supports  to 
the  beam  framework  must  be  withdrawn, 
and  the  framework  suspended — or,  rather, 
steadied — by  braces  across  the  top  of  the 
moulds.  The  moulds  having  been  erected 
and  fixed,  the  beams  must  be  concreted,  but 


THE   RETAINING   WALL1 

In  the  erection  of  a  retaining  wall  sup- 
porting the  roadway,  the  ground  must  be 
securely  shored  up  and  strutted,  and  pro- 
vision must  be  made,  when  erecting  the 
shuttering,  to  strut  the  ground  up  from  the 
centering,  but  the  shoring  must  not  in  the 
least  bear  on  the  retaining  wall.  For  the 


ERECTION    OF    A    BUILDING 


purpose  of  practical  illustration,  it  is  assumed 
that  the  vertical  bars  have  already  been 
placed  in  and  secured  to  the  foundation  beam 
and  slab,  which  has  been  specially  designed 
for  this  purpose.  These  bars  must  be  held 
upright  by  means  of  temporary  stays,  so 
that  they  do  not  become  twisted. 

The  wall  would  probably  be  about  6  in. 
thick  and  plumb  on  both  sides,  with  a 
horizontal  beam  half-way  up  and  a  capping 
beam  at  the  top.  There  will  also  be  counter- 
forts which  very  much  resemble  a  beam  in 
a  vertical  position  ;  they  will  probably  be 
about  2  ft.  6  in.  thick  at  the  base,  tapering 
to  the  width  of  the  capping  beam,  which 
may  be  about  18  in.  ;  moulds  will  be  con- 


structed for  these  similar  to  those  for  a 
column  in  a  wall.  The  reinforcements  will 
be  carefully  framed  up  while  the  concreting 
proceeds  as  the  vertical  rods  of  the  counter- 
forts will  have  already  been  fixed,  as  in  the 
case  of  the  vertical  bars  for  the  wall  itself. 
The  concreting  naturally  will  be  brought  up 
with  the  wall,  so  that  there  will  be  no  joint 
between  the  counterfort  and  wall.  Fig.  183 
shows  plan  of  a  retaining  wall  of  this  descrip- 
tion. For  a  wall  of  this  type,  it  is  advisable 
to  erect  the  centering  on  the  external  side 
to  the  whole  height  of  the  wall,  and  when 
this  is  done  the  vertical  bars  may  be  fixed 
to  it  at  their  proper  spacing,  thus  obviating 
the  use  of  temporary  stays.  The  horizontal 


Fig.    184. — Reinforcement  of  Retaining  Wall,  Royal  Insurance  Building 


REINFORCED    CONCRETE 


r 


Fig.  187.— Dia- 
gram showing 
the  above 
Retaining  Wall 
if  Built  with 
Brickwork 


Fig.  185  Fig.  186 

Figs.   185  and  186. — Reinforced  Concrete  Retaining  Wall  for  Royal  Insurance  Building 

bars  should  next  be  fixed  for  part  of  the  way 
up  and  securely  tied  at  their  intersections 
with  wire.  The  wall  is  now  ready  for  con- 
creting, and  the  shuttering — which  should  be 
about  3  ft.  high — should  be  erected  between 
the  studs,  which  are  generally  spaced  5  ft. 
or  6  ft.  apart.  The  concrete  should  be 
placed  in  the  wall  and  carefully  punned 
with  a  wall  punning  iron,  doing  the  whole 
length  of  the  wall,  if  possible,  in  one  day  ; 
the  next  day  another  3  ft.  should  be  con- 
creted, by  which  time  the  lower  shutters 
may  be  struck,  cleaned,  and  used  for  the 
.  third  3  ft.,  and  so  on  to  completion,  the 
remaining  horizontal  bars  being  fixed  as 
the  concreting  proceeds.  When  the  posi- 
tion for  the  beam  is  reached,  a  two-sided 
mould  must  be  securely  bracketed  out  from 
the  vertical  studs  supporting  the  shutters. 
The  beam  reinforcement  must  be  placed  in 
the  mould  and  connected  with  the  wall 
reinforcement,  the  concreting  following. 

Scaffolding  will  have  to  be  erected  to  com- 
plete the  wall,  and  it  should  be  constructed 
in  such  a  way  that  it  is  independent  of  the 


ERECTION    OF    A    BUILDING 


wall  except  that  it  may  be  tied  into  the 
studs  supporting  the  shuttering.  Care  must 
be  taken  not  to  shake  the  wall  in  erecting 
the  scaffolding,  because  the  concrete  in  the 
wall  is  still  "  green."  (Concrete  that  has 
set  but  is  not  properly  dry  is  in  a  condition 
known  as  "  green.")  The  striking  of  the 
outside  centering  should  not  be  done  for 
some  time,  as  the  wall  should  be  dry  right 
through  before  there  is  any  chance  what- 
ever of  any  weight  coming  on  it.  It  is  better 
to  avoid  all  unnecessary  risks.  In  a  few 
cases  where  the  wall  is  erected  close  against 
dug-out  ground  the  centering  has  to  remain 
because  it  cannot  be  got  at,  but  this  is  not 
often  so,  there  generally  being  enough  room 
between  the  outside  of  the  wall  and  the 
ground  for  a  man  to  get  in. 

A  retaining  wall  at  the  Royal  Insurance 
Building,  Lombard  Street,  London,  is  illus- 
trated by  Figs.  184  to  188,  the  diagrams 
being  comparative  and  showing  stability  and 


pressure  on  ground  of  reinforced  concrete 
and  brick  walls  under  identical  thrusts. 
The  space  available  did  not  allow  of  counter- 
forts. The  wall  is  26  ft.  6  in.  high  from 
underside  of  toe,  and  is  only  21  in.  thick 
at  the  bottom,  the  thickness  at  the  top 
being  10  in.,  there  being  two  offsets.  The 
wall  is  vertical  at  the  back,  the  heel  pro- 
jecting for  18  in.,  as  shown  in  Fig.  186.  The 
reinforcement  takes  the  form  of  vertical 
bars  and  horizontal  stirrups. 

COLUMNS 

While  the  retaining  wall  is  being  built  the 
carpenters  will  be  constructing  the  column 
moulds,  and  the  steel  workers  will  be  fixing 
up  the  columns.  Column  reinforcement  may 
be  done  in  at  least  two  ways  :  one  by  erect- 
ing the  bars  in  the  mould  and  putting  on  the 
ties  or  binding  as  the  concreting  proceeds ; 
the  other  by  framing  up  the  skeleton  of  the 
column  on  trestles  as  before  described  for 


Fig.  188. — A  Further  View  of  Retaining  Wall,  Royal  Insurance  Building,  in  Course  of  Erection 
10 


146 


REINFORCED    CONCRETE 


piles.  Probably  fewer  mistakes  are  liable  to 
be  made  in  the  second  method  with  regard 
to  the  exact  position  of  the  bars  and  the 
spacing  of  the  ties,  and  also  the  ties  will  be 
more  tightly  fixed  to  the  vertical  bars.  What 
is  meant  is  clearly  shown  in  Fig.  189  be- 
low. When  the  skeleton  columns  are  made 
they  should  be  carefully  stacked  in  a  place 
of  safety  where  the  ties  will  not  be  disturbed  ; 
this  precaution  is  often  neglected,  with  the 
result  that  the  steelwork  is  trampled  on  and 
displaced  as  shown  in  Fig.  189.  The  skele- 
ton columns,  having  been  placed  in  the 
moulds,  should  be  temporarily  fixed  by 
means  of  wood  templates  so  that  they  stand 
truly  plumb  on  all  four  sides  ;  and  it  should 
be  seen  that  the  space  between  the  outside 
of  the  vertical  bars  and  the  mould  is  correct. 
Before  any  concreting  is  permitted,  the 


CORftKT 


Fig.  189. — Cross  Sections  of  Columns  showing 
Right  and  Wrong  Methods  of  Placing 
Reinforcements 

clerk  of  works  should  inspect  all  columns  to 
see  that  there  is  no  dirt,  sawdust,  or  other 
rubbish  lying  in  the  bottom,  and  that  a 
good  grouting  has  been  poured  in  to  form 
the  joint  between  the  floor  and  column  or 
the  joint  between  the  column  under  the  floor 
and  the  column  above  it,  as  the  case  may  be. 
As  a  rule  the  vertical  bars  of  a  column  pro- 
ject through  the  floor  some  6  in.  or  8  in.,  so 
that  a  good  connection  can  be  made  with 
the  column  above  it.  The  concreting  of  a 
column  can  be  carried  out  in  one  of  two 
ways — either  by  pouring  in  the  concrete  from 
the  top,  or  concreting  and  punning  it  from 
the  bottom,  one  side  of  the  mould  being 
left  open  for  this  purpose  in  the  same 
manner  as  already  stated  for  concreting  the 
retaining  wall.  If  pouring  is  the  method 
employed,  no  one  pouring  should  fill  the 
column  to  a  greater  height  than  six  times 
its  diameter,  and  less  is  better.  In  filling 
columns  it  is  essential  that  the  concrete  is 
sufficiently  fluid  to  surround  the  bars  and 
ties,  and  it  should  be  rammed  with  a  long- 
handled  punning  iron.  Great  difficulty  is 
caused  by  the  presence  of  large  stones  in 
the  aggregate,  and  therefore  the  aggregate 
used  should  pass  through  a  |-in.  square  mesh. 


In  the  construction  of  buildings  of  several 
stories,  the  diameters  of  the  reinforced  con- 
crete columns  invariably  decrease  as  the 
building  gets  higher.  The  bars  of  one 
column  generally  project  through  the  floors, 
as  already  stated,  and  they  are  frequently 
bent  in  slightly  to  meet  the  bars  of  the 
column  above.  In  one  system  of  making 
the  joint,  a  sleeve  piece  is  fitted  to  each  bar 
of  the  lower  column,  and  the  bottom  ends 
of  the  bars  of  the  upper  column  enter  these 
sleeves.  By  another  system  the  bars  of  the 
upper  column  overlap  those  of  the  lower  one 
for  about  12  in.,  and  the  two  are  bound 
tightly  together  with  annealed  steel  wire  ; 
occasionally  the  binding  is  omitted. 

WALLS 

While  the  columns  are  going  up,  the  car- 
penters will  be  erecting  the  wall  shuttering  ; 
the  walls  will  probably  not  be  more  than 
6  in.  thick.  The  shuttering,  centering,  and 
concreting  for  an  ordinary  straight  wall  are 
worked  in  practically  the  same  fashion  as 
that  described  for  the  retaining  wall,  except 
that  openings  will  have  to  be  left  for  win- 
dows and  doors,  and  columns  (incorporated 
in  the  walls)  for  supporting  the  beams  and 
lintels  will  be  erected  practically  in  the  same 
way  as  described  for  the  counterforts  to 
the  retaining  wall.  Extra  reinforcement  is 
generally  placed  round  the  openings  ;  it  may 
consist  of  a  bar  about  f  in.  to  1  in.  in  dia- 
meter, bent  to  fit  the  opening  with  the  ends, 
where  they  meet,  overlapping  or  hooked 
through  one  another  ;  and  both  the  vertical 
and  horizontal  bars  in  the  wall  are  bent  or 
hooked  round  the  bar  as  shown  photo- 
graphically in  Fig.  527  (p.  324). 

In  fixing  the  reinforcing  bars  in  the  wall, 
close  observation  should  be  kept  to  see  that 
the  spacing  or  pitch  of  all  bars  agrees  with 
the  specification  and  drawings. 

Columns,  floors  and  walls  in  course  of 
erection  at  the  Money  Order  Office,  Hol- 
loway,  London,  are  illustrated  by  Figs.  193 
and  194  (pp.  148  and  149). 

STAIRCASES 

During  the  erection  of  the  walls  and 
columns  provision  will  have  to  be  made  for 
the  erection  of  the  staircase,  which  will 
either  be  erected  in  situ  or  built  up  with 
separate  treads  or  on  reinforced  concrete 
stringers  constructed  for  their  reception. 
In  the  present  instance  it  is  proposed  to 
describe  the  former  system. 


ERECTION    OF    A    BUILDING 


147 


The  Factory  Act  does  not  allow  a  straight 
flight  to  contain  more  than  fifteen  treads  in 
a  single  going  ;  therefore,  in  most  cases, 
when  there  is  a  height  of  from  12  ft.  to  14  ft. 
between  the  floors,  two  flights  must  be 
constructed  between  two  floors,  connected 
by  a  half-space  landing,  with  a  fireproof 
division  wall,  as  shown  in  Fig.  190.  In  a 
staircase  of  this  type,  treads  are  sup- 
ported on  a  reinforced  concrete  carriage  slab, 
on  which  they  are  moulded,  the  half-space 
landing  being  formed  in  the  same  way  as 


CROSS  SECTION 


PLAN 


Figs.    190   and    191. — Horizontal    and    Vertical 
Sections  through  Typical  Factory  Staircase 

the  carriage.  The  centering  for  the  carriage 
slab  and  landing  must  be  erected,  with 
moulds  for  beams  supporting  the  carriage 
and  landings.  The  reinforcement  is  then 
laid  in  position  in  the  same  way  as  described 
for  a  floor,  and  moulds  are  then  made  for 
forming  the  treads  and  fixed  in  the  right 
position,  so  that  the  thickness  (say,  4  in.) 
of  the  slab  carriage  can  be  maintained  during 
the  building  of  the  treads.  In  some  cases 
the  treads  are  reinforced  with  rods  laid  in 
longitudinally  and  hooked  into  the  walls. 


Of  course,  the  walls  surrounding  the  stairs 
must  be  carried  up  at  the  same  time  as  the 
stairs.  The  concreting  is  begun  at  the 
lowest  tread  and  worked  upwards,  being 
carefully  punned  the  whole  time.  Stairs  of 
this  kind  are  generally  finished  in  granolithic 
about  1  in.  thick,  a  thinner  layer  invariably 
cracking  and  peeling  off.  The  granolithic 
for  the  riser  must  necessarily  be  applied  to 
the  form  or  mould  before  the  concrete  is 
deposited  ;  that  for  the  treads  is  done 
afterwards,  but  the  sooner  it  is  done  the 
better  will  be  the  key  between  the  grano- 
lithic and  the  concrete.  Treads  are  generally 
finished  off  by  being  grooved  with  a  grooving 
roller,  so  as  to  prevent  their  becoming 
smooth  and  slippery.  Figs.  190  and  191 
show  a  typical  cross  section  through  a 
staircase  of  the  type  described  above. 

INTRODUCING  THE  REINFORCEMENTS 
INTO   BEAM   MOULDS 

As  already  suggested  in  the  case  of  piles, 
the  reinforcement  for  beams,  columns,  canti- 
levers, etc.,  can  be  introduced  in  either  of 
two  ways.  One  is  to  build  or  frame  the  bars 
up  in  the  moulds,  and  the  other  is  to  frame 
them  up  first  and  then  to  deposit  them 
bodily  in  the  moulds.  Where  members  are 
loose  and  not  tied  to  one  another,  the  rein- 
forcement must  necessarily  be  built  up  in  the 
moulds.  Much  care  and  patience  must  be 
exercised  in  this  part  of  the  work.  In 
building  a  beam,  the  bottom  bars  must  be 
kept  at  the  proper  distance  from  the  bottom 
and  sides  of  the  mould,  and  this  can  be 
effected  by  using  reinforced  concrete  blocks 
hollowed  out  as  in  Fig.  192  to  receive  the 


STEEL  ROD 


APPROX 


Fig.    192. — Hollowed-out    Concrete    Block    to 
Facilitate  Spacing  of  Bars 

bars  at  the  correct  spacing,  two  bars  for 
each  beam ;  the  blocks  are  laid  in  the 
bottom  of  the  mould,  and  when  the  beam 
is  concreted  up  the  new  concrete  will  adhere 
to  the  other  concrete.  One  firm  of  con- 
tractors always  adopts  this  practice,  which 
it  has  found  to  be  effective,  although  it 
might  give  the  impression  of  introducing  a 
source  of  weakness.  Another  way  is  to 


'49 


REINFORCED    CONCRETE 


place  blocks  of  wood  of  the  right  thickness 
in  the  bottom  of  the  mould,  and  to  lay  the 
bars  on  them  ;  these,  of  course,  have  to  be 
removed  when  there  is  sufficient  concrete 
in  the  mould  to  hold  the  bars  in  the  right 
position  ;  as  there  is,  of  course,  a  risk  of 
the  wooden  blocks  being  left  in  the  mould, 
the  foreman  must  watch  the  work  very 
carefully. 

In  all  cases  of  building  a  beam  in  a  mould, 
the  stirrups  have  to  be  placed  in  first,  and 
their  spacing  must  be  correctly  marked  on 
the  centering,  each  stirrup  being  adjusted 
after  a  little  concrete  has  been  deposited 
to  hold  it  in  place.  The  use  of  loose  stirrups 
necessitates  especial  care  in  this  particular, 
since  it  is  easy  for  a  careless  workman  to 
knock  about  the  stirrups  in  all  directions 
during  the  process  of  punning. 


VERTICAL   5UPPORT 
HORIZONTAL  SUPPORT 


Fig.  195. — Method  of  Supporting  Reinforcement 
in  Beam  Mould 

A  method  of  supporting  reinforcements 
in  beam  moulds  before  concreting  is  shown 
in  Fig.  195. 

Difficulty  is  often  met  with  at  the  inter- 
section of  beams  over  a  column,  there  being 
trouble  in  getting  the  bars  into  their  proper 
positions,  and  particularly  in  causing  them 
to  maintain  their  proper  distance  from  the 
sides  of  the  moulds.  Sometimes  the  neces- 
sary adjustment  is  made — and  made  badly 
— by  the  aid  of  a  sledge  hammer  and  a  con- 
siderable show  of  temper !  The  most  effec- 
tive method  is  to  bend  slightly  one  or  two  of 
the  bars  at  their  ends  so  enabling  them 
to  overlap  properly. 

In  vertical  work,  very  long  bars  are  some- 
times used,  and  these  will  always  need 
to  be  properly  supported  temporarily  to 
prevent  their  being  twisted  and  entangled. 
Long  straight  bars  can  safely  be  bundled 
together  and  fixed  to  a  suitable  support, 
but  when  the  bars  have  been  bent  to  a  par  • 


ticular  form,  they  must  be  supported  inde- 
pendently. 

For  the  fixing  of  stirrups  and  ties  of  all 
kinds  when  framing  up  a  member  in  its 
mould,  their  positions  must  always  be 
clearly  marked  in  pencil  on  the  mould  itself, 
so  that,  as  the  work  is  brought  up,  the  ties 
can  be  fixed  in  their  proper  places. 

FORMING  SKELETON  REINFORCEMENTS 

For  framing  a  member,  all  the  bends  must 
first  be  made,  the  bent  bars  being  then  laid 
on  trestles,  framed  up  in  their  proper  posi- 
tions on  wooden  templates,  and  then  tied 
temporarily  in  their  places  while  the  bind- 
ings or  stirrups,  etc.,  are  being  fixed  to  them. 
In  the  case  of  a  column  bound  spirally,  the 
bars  are  placed  in  wooden  templates,  in 
which  is  a  central  hole  through  which  passes 
a  rod  to  act  as  a  spindle ;  the  ends  of  this 
rod  are  rested  on  the  trestles,  so  that  the 
whole  framework  can  be  made  to  revolve. 
(This,  however,  does  not  apply  to  spiral  bind- 
ing as  designed  for  the  Considere  system, 
for  which  the  binding  must  be  carried  out 
on  a  drum  first  and  threaded  on  to  the  bars 
afterwards,  as  described  for  the  Considere 
pile.)  The  binding  is  then  securely  fixed  on 
a  bar  of  the  framework,  and  so  wound  round 
and  round  while  the  framework  is  made  to 
revolve.  The  pitch  of  the  binding  is  marked 
clearly  on  one  of  the  rods.  When  a  length 
of  binding  is  exhausted,  the  end  is  turned 
tightly  round  a  bar  and  another  length  is 
begun  on  the  bar  before  that  on  which  the 
first  length  finishes.  The  binding  is  con- 
tinued in  a  like  manner  till  the  column  is 
complete,  but  it  needs  to  be  securely  tied 
on  with  annealed  wire  at  every  alternate 
bar  to  obviate  slipping.  When  the  binding 
is  in  the  form  of  rings  the  work  is  done  in 
the  same  manner  as  described  for  piles  (see 
p.  133). 

For  framing  up  a  beam,  the  stirrups  are 
cut  and  bent  and  then  threaded  on  to  the 
beam  framework,  the  bars  of  which  are 
temporarily  placed  in  their  correct  positions 
on  templates,  as  in  the  case  of  a  column. 
The  spacing  of  the  stirrups  is  marked  on 
one  of  the  bars  running  the  whole  length  of 
the  beam,  and  the  stirrups  are  tied  to  the 
main  bars  with  annealed  wire  accordingly. 
All  independent  members  can  be  so  framed 
up  ready  for  deposition  or  placing  in  the 
moulds,  and  if  not  used  at  once  they  should 
be  carefully  stacked  away  in  a  place  where 
they  will  not  be  subjected  to  hard  knocks. 


ERECTION    OF    A    BUILDING 


The  bending  of  small  bars  round  larger 
bars  is  best  done  with  the  aid  of  the  wrench 
previously  described. 

Unfortunately  it  is  easy  for  a  man  with  a 
shovel,  hammer,  or  a  punning  iron  to  cause 
any  amount  of  mischief  if  allowed  to  use 
these  tools  carelessly.  Extreme  care  should 
be  taken  in  the  fixing  of  reinforcements  over 
points  of  support  and  also  in  the  making  of 
connections. 

MAKING   A   FLOOR 

The  very  first  thing  to  be  considered  in 
the  erection  of  a  floor  is  the  centering  or 
falsework.  Moulds  or  troughs  will  be  needed 
for  the  reception  of  the  main  and  secondary 
beams,  and  they  will  need  to  be  securely 
strutted  from  the  floor  beneath.  They  must 
be  in  true  alignment  and  of  sufficient  strength 
to  bear  the  weight  of  the  work  that  will 
come  upon  them.  The  wood  sheeting  is 
laid  between  the  beams  to  support  the 


round  rounds  was  described  in  connection 
with  the  foundation  slab.  For  a  suspended 
or  supported  floor,  the  reinforcing  will  be 
carried  out  in  the  same  way,  but,  of  course, 
centering  is  holding  up  the  floor,  whereas  a 
foundation  slab  rests  on  the  ground.  The 
reinforcement  may  take  the  form  of  bars 
and  rods  or  of  a  continuous  mesh  made  with 
small  diameter  rods  or  from  sheet  steel.  But 
before  any  slab  reinforcement  is  laid,  the 
beams  must  be  arranged  for ;  the  beam 
reinforcements  must  either  be  built  up  in  the 
moulds  or  put  together  as  frameworks  and 
afterwards  laid  in  them.  Wooden  blocks 
should  be  laid  across  the  mould  to  support 
the  top  or  compression  bar  of  the  beam  ; 
and  wooden  strips  should  be  inserted  on 
each  side  to  maintain  the  correct  distance 
between  the  reinforcement  and  the  faces 
of  the  form ;  but  great  care  must  be  taken 
that  all  these  blocks  and  strips  are  removed 
as  soon  as  there  is  sufficient  concrete  in  the 


%3fa?^t'?t:%:rt^ 

' "  "  '          ^        •       '  ' — " 


Fig.   196. — Typical  Floor  with  Continuous  Mesh  Reinforcement 


weight  of  the  floor  slab  and  of  the  men 
working  on  it.  Descriptions  of  the  various 
methods  of  constructing  floor  centering  will 
be  found  in  the  next  chapter. 

Before  any  steel  reinforcement  is  placed 
in  or  on  the  floor  centering,  all  the  moulds 
for  beams  must  be  thoroughly  cleared  of  all 
shavings,  sawdust,  dirt,  or  other  rubbish 
which  always  accumulates  during  the  erec- 
tion of  the  centering. 

For  the  purpose  of  preventing  concrete 
adhering  to  the  centering,  it  is  advisable  to 
close  the  pores  of  the  wood  by  thoroughly 
wetting  all  internal  surfaces  just  previous 
to  depositing  the  concrete.  When  a  better 
finish  is  required,  the  boards  may  be  advan- 
tageously limewashed  or  treated  with  soft 
soap  before  the  steel  is  placed  in  position  or 
the  concreting  started ;  this  makes  the 
centering  easier  to  remove  and  gives  the 
work  an  improved  appearance. 

The  reinforcing  of  a  floor  with  ordinary 


moulds  to  keep  the  bars  in  place.  Fig.  196 
shows  a  beam  mould  with  the  reinforcement 
adjusted  by  the  means  just  described. 

Concreting  must  not  be  started  until  all 
the  reinforcement  is  complete  and  correct. 
Obviously,  mistakes  cannot  be  rectified  once 
the  concrete  has  been  introduced. 

The  concreting  is  next  carried  out  in 
exactly  the  same  way  as  described  for  the 
foundation  slab  beams,  the  concrete  being 
taken  up  to  the  level  of  the  top  of  the  moulds 
only,  and  left  as  rough  and  as  rugged  as 
possible,  so  as  to  make  a  good  key  for  the 
slab  concrete,  which  is  added  after  the  slab 
reinforcement  has  been  laid. 

Continuous  wire  mesh  is  generally  sup- 
plied in  large  rolls  containing  from  150  ft. 
to  300  ft.  run,  and  varying  from  3  ft.  to 
6  ft.  in  width.  For  laying  this  to  form  a 
floor  reinforcement,  the  rolls  are  hoisted 
on  to  the  centering  and  placed  side  by  side 
at  one  end.  The  roll  is  then  undone  and 


152 


REINFORCED    CONCRETE 


the  end  taken  into  the  wall  (see  B,  Fig.  196) 
and  fixed  with  staples  to  the  centering,  the 
mesh  being  next  unrolled  over  the  entire 
length ;  temporary  blocks  are  placed  on 
the  beams  (see  A),  over  which  it  passes  to 
bring  up  the  mesh  to  the  level  required  over 
the  supports,  as  shown  in  the  engineer's 
drawings.  The  correct  length  having  been 
cut  off,  the  free  end  of  the  mesh  is  fixed 
into  the  wall  (see  B).  The  other  rolls  are 
laid  in  the  same  way,  side  by  side,  the  space 
between  them  being  determined  by  the  pitch 
of  the  main  or  tension  strands  of  the  mesh  ; 
for  example,  if  the  pitch  of  the  mesh  is 
4  in.  the  selvedge  of  one  roll  is  4  in.  from 
the  selvedge  of  the  other  roll.  Reinforce- 
ment of  this  type  can  be  quickly  laid,  since 
there  is  no  spacing  and  tying  of  rods  or  bars. 
When  sheet  mesh  instead  of  the  wire 
mesh  is  used,  first  of  all  it  is  laid  flat  on 


The  concreting  of  suspended  floors  is 
done  in  the  same  manner  as  described  for 
the  foundation  slab  in  the  earlier  part  of 
this  chapter  ;  but  it  may  be  well  to  give  in 
this  place  some  further  particulars  of  the 
process  of  concreting  as  applied  to  floor  con- 
struction. The  concrete  should  not  be  so 
thin  that  the  water  may  run,  and  carry  with 
it  the  cement,  through  any  openings  and 
cracks  in  the  centering,  thus  causing  the 
slab  when  dry  to  have  a  honeycomb  struc- 
ture. The  more  water  used  in  mixing 
concrete,  the  greater  is  the  shrinkage,  but 
workmen  are  more  likely  to  make  con- 
crete over-wet  than  over-dry  because  of  the 
greater  ease  in  handling  and  the  less  punning 
that  will  be  required.  The  concrete  should 
be  sufficiently  moist  to  be  worked  properly 
into  the  moulds  and  round  the  reinforce- 
ment. 


1-2    °F  SPAtS       I 

_|^g^jfj^^  - 


Fig.   197. — Typical  Floor  with  Sheet  Mesh  Reinforcement 


the  centering.  The  sheets  may  butt  over  the 
supports,  for  here  there  is  an  additional  sheet 
laid  near  the  top  of  the  slab,  these  additional 
sheets  extending  over  '2  (one-fifth)  of  the 
span  on  each  side  of  the  centre  of  the  sup- 
port. Fig.  197  shows  a  cross  section  of 
this  type  of  floor,  and  clearly  illustrates 
what  is  meant  by  the  above.  Other  mesh- 
reinforced  floors  are  illustrated  in  a  later 
chapter  devoted  to  the  various  commercial 
systems. 

In  laying  an  expanded  metal  floor,  the 
sheets  must  be  so  laid  on  the  centering  that 
the  diagonal  strands  in  the  sheets  all  slope 
in  one  direction,  otherwise  the  ends  of  the 
sheets  will  not  properly  key  into  one  another 
where  they  overlap.  If  the  sheets  bulge 
slightly  when  laid  in  position,  a  few  nails  or 
staples  should  be  driven  into  the  centering 
to  hold  the  metal  down  ;  when  the  center- 
ing is  struck,  the  staples,  etc.,  will  remain  in 
the  concrete,  but  can  easily  be  clipped  off. 
As  already  stated,  while  the  concreting  of 
this  type  of  floor  is  proceeding,  the  mesh 
should  be  lifted  and  shaken  when  there  is 
a  layer  of  concrete  f  in.  or  1  in.  thick  on 
which  it  can  rest. 


As  far  as  is  practicable,  the  concreting  of 
a  floor  slab  over  a  complete  area  should  pro- 
ceed at  one  time  so  as  to  ensure  the  work 
being  monolithic  ;  but  when  a  whole  floor 
cannot  be  completed  in  one  day  the  work 
should  be  divided  as  follows : — For  a  main 
beam,  the  concreting  must  be  carried  to  the 
centre  of  a  column.  A  secondary  beam  must 
be  filled  up  to,  and  at  the  same  time  as,  the 
main  beam  to  which  it  is  connected.  In  a 
slab,  the  concrete  must  be  carried  to  the 
centre  of  a  beam.  A  column  must  always  be 
carried  to  its  full  height.  Walls  should  be 
worked  along  a  complete  length,  and  any 
stoppage  of  work  should  occur  at  a  horizontal 
level. 

Concrete  spread  on  a  floor  should  be  carried 
forward  on  an  even  line  across  the  whole 
width  of  a  bay. 

It  will  be  readily  understood  that  the 
remainder  of  the  floors,  columns,  and  walls 
in  the  factory  will  be  carried  up  to  the  flat 
roof,  which  is  exactly  the  same  as  a  floor, 
except  that  there  may  be  openings  left  in 
it  for  lantern  lights,  that  the  concrete,  in- 
stead of  being  laid  level,  will  be  laid  to 
falls,  that  gutters  will  be  hollowed  out  to 


ERECTION    OF    A    BUILDING 


take  the  water,  and  that  a  parapet  will  be 
erected  in  continuation  of  the  walls. 

THE   ERECTION    OF    CENTERING 

From  what  has  already  been  said,  it  will 
be  seen  that  as  the  work  proceeds  all  the 
centering  will  have  to  be  struck,  cleaned, 
and,  if  possible,  re-used — work  in  which  the 
foreman  will  need  all  his  ingenuity  to  effect 
legitimate  economy,  which  is  a  different 
thing  from  scamping.  The  foreman  must 
arrange  that  the  centering  is  always  erected 
in  advance — well  in  time  for  the  placing  of 
the  reinforcement.  He  should  strike  the 
centering  in  plenty  of  time  for  it  to  be  pro- 
perly scraped  and  cleaned  before  refraining 
it,  but  everything  he  does  must  be  consistent 
with  absolute  safety.  In  the  case  of  floors 
of  large  areas,  the  centering  for  the  height 
of  and  including  two  floors  should  be  erected 
as  soon  as  possible,  so  that  by  the  time  half 
of  the  second  floor  concreting  is  done,  the 
centering  from  the  first  floor  can  be  struck 
and  used  for  the  next ;  but  he  must  exer- 
cise extreme  caution  to  see  that  the  bottom 
planks  of  the  beam  forms  and  their  supports 
are  not  removed  until  it  is  safe  to  do  so. 
All  slabs  need  to  be  temporarily  supported 
after  the  removal  of  the  sheeting.  The 
sanction  of  the  architect,  engineer,  or  clerk 
of  works  should  always  be  obtained  before 
any  striking  is  begun. 

The  ease  with  which  centering  is  removed 
is  a  good  criterion  of  the  quality  of  its  con- 
struction ;  but,  however  well  it  has  been  put 
together,  it  is  necessary  to  employ  a  number 
of  careful  men  to  strike  it.  At  least  five 
or  six  should  be  detailed  off  for  this  work, 
and  it  should  be  definitely  explained  to 
them  that  the  concrete  is  not  yet  thoroughly 
set,  and  that  they  must  exercise  care  to 
prevent  the  jarring  or  chipping  off  of  arrises, 
etc. 

Opinions  vary  as  to  the  period  that  should 
elapse  before  the  false-work  is  removed ; 
but  the  following  is  safe  practice. 

Floor  slabs  and  beams. — The  centering  may 
be  loosened  in  seven  or  eight  days  and 
struck  in  ten,  except  the  bottom  planks 
and  supports  to  beams. 

Wall  panels. — The  strutting  can  be  re- 
moved in  two  or  three  days  in  moderate 
weather ;  the  sooner  they  are  struck,  the 
better  chance  the  wall  has  of  drying  out  and 
hardening. 

Columns,  etc. — Three  to  five  days  under 
similar  conditions  to  the  above. 


It  is  as  well  to  loosen  all  centering  before 
striking,  in  order  to  prevent  the  concrete 
from  sticking  to  the  boards  ;  moreover,  the 
air  circulating  through  will  harden  the  sur- 
faces of  the  concrete,  which  will  not  be  so 
liable  to  be  chipped. 

All  suspended  members  must  be  left 
supported  by  temporary  shores  placed  at 
judiciously  chosen  points,  and  left  for  any- 
thing from  fifteen  to  thirty  days.  Where 
support  is  most  needed  is  at  the  centres  of 
beams  and  floor  slabs.  On  no  account  must 
any  weight  be  allowed  on  any  floor  or 
against  any  wall  for  at  least  three  weeks 
after  the  removal  of  the  centering,  except 
that  of  the  timber  for  the  floor  above  and 
of  the  men  worldng  on  it. 

FINISHING   CONCRETE   SURFACES 

The  finishing  of  concrete  is  considered  in 
detail  on  pp.  241  to  255,  but  it  is  desirable 
in  this  chapter  to  complete  the  discussion 
of  the  practical  work  of  erecting  a  reinforced 
concrete  building  by  giving  a  few  notes  on 
the  subject.  Concrete  walls  can  be  cleaned 
down,  all  excrescences  removed,  and  hollows 
filled  in,  and  then  whitewashed — this,  of 
course,  applying  only  to  buildings  of  the 
warehouse  type.  They  can  be  decorated 
with  plaster  in  the  same  way  as  any  other 
building,  but  for  this  purpose  the  concrete 
ought  to  be  hacked  or  scored  to  form  a 
key  for  the  plaster.  In  this  hacking,  great 
care  must  be  exercised,  because  a  careless 
man  with  a  bush  hammer  or  similar  imple- 
ment can  do  much  damage,  although  this 
may  not  be  visible  to  the  eye.  The  hacking 
must  be  done  while  the  concrete  is  green — 
that  is  to  say,  friable.  Take  the  case  of  a 
reinforced  concrete  wall  4  in.  thick  that  is 
to  be  plastered  inside  and  finished  in  stucco 
outside,  necessitating  hacking  on  two  sides. 
It  is  obvious  that  a  man  giving  the  green 
concrete  wall  hard  knocks  with  a  hammer 
will  be  causing  infinite  mischief  by  dis- 
integrating the  concrete,  and  thus  destroy- 
ing its  adhesive  strength  and  monolithic 
nature. 

THE     FIXING     OF      WINDOW     FRAMES, 
DOOR   FRAMES,    ETC. 

Provision  always  has  to  be  made  for  the 
fixing  of  frames,  etc.,  to  the  openings  left 
for  them ;  an  effective  method  is  to  insert 
screws  in  their  proper  positions  in  the  con- 
crete, with  their  worms  bound  round  with 
wire,  and  when  the  concrete  is  sufficiently 


154 


REINFORCED    CONCRETE 


set,  to  give  each  screw  a  turn  ;  it  will  be 
found  that  the  wire  is  a  perfect  hold  for 
the  screw,  and  the  frames,  etc.,  can  be  fixed 
without  difficulty. 

The  Craig  screw-bore  (see  Fig.  198)   is  a 
great  convenience  when  it  is  desired  to  attach 

fittings,  wood 
finishes,  etc.,  to 
the  concrete. 
The  method  of 
using  it  is  first  to 
insert  a  master 
bolt,  as  illus- 
trated, through 
a  hole  made  in 
the  side  of  the 
form ;  the  wire 
screw  -  bore  is 
Fig.  198. — Craig  Screw-bore  then  screwed  on 

to  the  bolt,  the 

reinforcement  placed  in  position,  and  the 
ends  of  the  screw-bore  wound  around  the 
reinforcement  if  this  is  thought  necessary. 
Before  removing  the  forms,  the  bolt  is  in- 
screwed,  thus  leaving  the  screw-bore  cor- 
rectly placed,  and,  of  course,  most  rigidly 
held,  and  forming  a  fixed  nut  into  which  a 
screw  threaded  the  same  as  the  master 
bolt  is  easily  inserted. 


STEEL    CORNER    BARS 

Steel  kerbs  or  corner  bars  built  into  con- 
crete steps  answer  three  purposes  :  as  regards 
appearance  they  give  a  finish  to  the  con- 
struction, they  provide  a  durable  edge  at 
the  very  place  where  durability  is  most 
needed,  and  to  a  great  extent  they  act 
as  reinforcement.  United  States  firms  who 
have  specialised  in  these  kerbs  apparently 
agree  that  the  form  shown  in  Fig.  199  is  the 
best  for  the  purpose,  the  illustration  showing 
the  Wainwright  kerb,  which  is  made  of  such 
a  shape  as  to  be  well  anchored  into  the  con- 
crete. The  bars  are  made  in  various  sizes 
for  various  widths  of  steps.  The  appearance 
of  a  staircase  fitted  with  such  kerbs  is  shown 
in  Fig.  200.  An  interesting  application  of 
the  same  idea  is  shown  in  Fig.  201,  which 
is  a  section  through  a  reinforced  concrete 
column,  the  four  edges  of  which  are  built  up 
with  corner  bars  made  of  mild  steel  and 
well  galvanised.  In  the  illustration,  A  in- 
dicates the  corner  bars  and  B  spreaders  made 
of  stamped  steel  1  in.  wide  and  |  in.  thick, 
which  maintain  the  corner  bars  in  proper 
position  while  the  concreting  proceeds. 
Should  additional  reinforcement  be  required, 
it  may  take  the  form  of  upright  bars,  c, 
with  wire  links,  D. 


Fig.  202.— Ebco 
Corner  Bar 


Fig.  199.— The  Wain- 
wright Steel  Kerb 


Fig.  200.— Steps  with  Steel  Kerbs 

. 

A  method  that  is  more  generally  employed 
is  to  insert  breeze  bricks  at  various  places 
round  the  opening,  so  that  the  frames  can 
be  screwed  into  them. 


Fig.  201. — Section  through  Column  with  Steel 
Corner  Bars 

Another  type  of  corner  bar — the  Ebco — 
also  provides  a  rounded  corner,  but  is  made 
with  anchors  spaced  approximately  17  in. 
apart,  as  shown  in  Fig.  202. 


Forms   and   Centerings 


Design. — Owing  to  the  fact  that  the  con- 
struction of  the  forms,  or  shuttering  and 
centering,  or  horsing  accounts  for  a  very 
large  proportion  of  the  total  cost  of  rein- 
forced concrete  work,  economy  in  the  false- 
work is  most  desirable,  and  the  only  proper 
way  of  ensuring  this  is  by  the  adoption  of  a 
correct  design  in  the  first  place.  This  is 
recognised  in  the  United  States,  for  the 
scheme  of  the  centering  is  usually  decided 
in  the  office  before  the  work  is  begun.  Good 
design  in  centering  is  evidenced  by  stability 
and  rigidity,  ease  of  removal  or  striking,  and 
facility  of  rapid  re-erection.  In  the  case  of 
a  building  which  is  regular  in  form  and  con- 
tains a  number  of  stories  which  are  alike 
except  as  regards  dimensions,  thickness  of 
floor  and  scantlings  of  beams  and  columns, 
a  well-designed  centering  may  be  used 
many  times  before  the  roof  is  reached.  In 
the  case  of  members  of  unusual  shape,  it  is 
only  by  means  of  proper  design  that  excessive 
waste  is  avoided. 

Centering  should  be  designed  in  such  a 
way  that  it  may  be  struck  readily  without 
jarring  or  hacking  the  arrises  off  the  con- 
crete, and  to  attain  this  object  as  few  nails 
as  possible  should  be  used,  thus  necessitating 
close  supervision,  for  the  carpenter  never 
loses  an  opportunity  of  driving  a  nail.  Beam 
and  column  moulds  and  wall  panelling  should 
be  so  made  that  the  various  members  fit 
into  one  another  without  nailing,  the  whole 
being  made  secure  by  bolting,  wedging,  and 
like  means. 

The  contractor  is  tempted  to  employ  too 
light  timber,  with  consequent  risk  to  the 
stability,  but,  on  the  other  hand,  too  heavy 
timber  is  unnecessarily  expensive  and 
awkward  to  handle.  The  timber  used  should 
be  strong  enough  to  take,  without  appreci- 
able deflection,  any  load  imposed  upon  it  by 
the  reinforced  concrete,  the  men  working 
on  it,  and  the  tamping.  Where  practicable, 
the  scantlings  of  the  timbers  to  be  used  will 
be  given  in  this  chapter. 

It  is  essential  to  maintain  the  perfect 
alignment  of  beams,  and  to  see  that  all 
vertical  members  are  truly  plumb,  for 
reasons  that  are  obvious.  Further,  all 


moulds  should  be  carefully  and  sufficiently 
strutted  so  as  to  prevent  sagging  or  bulging 
after  the  concrete  has  been  deposited  ;  and 
all  joints  should  be  made  tight  enough  to 
prevent  the  thinner  part  of  the  concrete 
running  through,  but  at  the  same  time  allow- 
ances must  be  made  so  that  when  the  boards 
swell  with  the  moisture  they  can  give  but 
not  bulge. 

It  is  advisable  to  add  angle  fillets  to  beam 
and  column  moulds  so  as  to  form  a  chamfer 
on  the  concrete,  (a)  thus  obviating  sharp 
arrises  which  tend  to  knock  off  when  remov- 
ing the  centering  and  so  reducing  the  amount 
of  patching  up  to  be  done  afterwards ; 
(6)  making  spalling  less  easy  in  case  of  fire  ; 
and  (c)  giving  a  finished  appearance  to  the 
work.  It  is  a  good  plan  to  construct  all  beam 
moulds  with  a  camber  at  the  centre  of  at 
least  \  in. ;  the  beam  will  not  come  out 
straight  on  the  removal  of  the  centering, 
but  in  any  case,  the  camber  looks  better 
than  a  slight  bulge  in  the  middle,  and  it  is 
well  known  that  ancient  Greek  architects 
cambered  all  horizontal  lines  to  make  them 
look  straight  to  the  eye. 

Failures  may  often  be  quite  as  much 
attributable  to  weak  false-work  or  to  the 
removal  of  the  false-work  before  the  con- 
crete is  sufficiently  set,  as  to  such  a  cause 
as  bad  design.  Since  any  settlement  that 
may  take  place  when  the  concrete  is  newly 
made  may  easily  prove  disastrous,  rigidity 
of  the  forms  becomes  essential. 

The  forms  and  centerings  described  in  this 
chapter  are  taken  from  actual  examples  in 
practice,  this  course  being  considered  prefer- 
able to  the  discussion  of  generalities. 

SIZES  OF  TIMBER 

Beams.  —  Bottom  plank,  2  in.  to  2f  in. 
thick.  Sides  or  cheeks,  1J  in.  for  secondary 
beams  and  If  in.  to  2  in.  for  main  beams. 

Columns  or  'pillars. — Sides,  If  in.  to  2  in. ; 
corner  studs,  4  in.  by  3  in.  and  5  in.  by 
2  in. 

Slab. — 1-in.  boarding,  maximum  spacing 
for  1-in.  boarding,  2  ft. ;  maximum  spacing 
for  If-in.  boarding,  4  ft. ;  maximum  spacing 
for  2-in.  boarding,  5  ft. 


156 


REINFORCED    CONCRETE 


Slab  beams. — 6  in.  by  3  in.,  spaced  2  ft.  to 
2  ft.  6  in.  apart,  according  to  nature  of  slab. 

Walls. — Kunners,  9  in.  by  3  in. 

Mr.  Sandford  E.  Thompson  gives  the 
following  safe  loads  for  timber  struts  in 
forms  for  floor  construction  : — 


Strut 
Length 

3"  x  4" 

4"  x  4" 

6"  x  6" 

8"x8" 

Ib. 

Ib. 

Ib. 

Ib. 

14  ft. 

500 

700 

900 

1100 

12  ft. 

600 

800 

1000 

1200 

10  ft. 

700 

900 

1100 

1200 

8  ft. 

850 

1050 

1200 

1200 

6  ft. 

1000 

1200 

1200 

1200 

The  following  tables  have  been  carefully 
worked  out  and  will  serve  as  a  useful  guide  : 


bridges ;  (9)  place  new  posts  under  girders  near 
beams  with  cross  heads  running  along  girder 
bottoms  in  same  position  as  original  post ; 
(10)  prop  up  slab  joists  temporarily  as  near 
beams  as  possible;  (11)  draw  nails  holding 
beam  sides  to  beam  bottom ;  (12)  lower  wedges 
under  posts  supporting  beams  and  turn  cross 
heads  lengthwise  and  wedge  up  again ; 
(13)  remove  beam  sides ;  (14)  remove  girder 
sides ;  (15)  remove  slab  sheeting  and  joists. 

SETTING  OUT 

The  correct  position  of  the  forms  can  result 
only  from  extremely  careful  setting  out  on 
a  proper  system  allowing  of  the  main  lines 
being  referred  to  during  the  progress  of  the 
work.  "  Horses  "  are  made  with  3-in.  by 
2-in.  stakes  driven  into  the  ground,  with 
floor  boards,  from  6  in.  to  9  in.  wide,  and  any 
convenient  length  up  to  a  few  feet,  nailed  to 


TABLE  SHOWING  DISTANCES  BETWEEN  SUPPORTS  TO  BOARDS,  &c.,  FOR  FALSE-WORK 


Boards, 
Planks,  &c. 

1" 

U" 

11" 

If" 

2" 

2i" 

2J" 

3" 

Distances 

2'0" 

2'  6" 

3'0" 

3f  />  n 
6 

4'  0" 

4'  6" 

5'0" 

5'0"-6'0" 

TABLE  SHOWING  DISTANCES  APART  FOR  VARIOUS  SCANTLINGS  IN  STRUTTING 
AND  SHORING  TO  FALSE-WORK 


Scantling 

(varies  with 
height) 

4"  x  2" 

4J"  x  3" 

6"  X  2" 

7"  X  2" 

8"  x  2J" 

9"  x  3" 

10"  X  5" 

12"  x  6" 

Distances 

2'0" 

3'0"-3'6" 

2'6"-3'0" 

3'0"-3'6" 

4'0" 

4'6"-5'0" 

5'6"-6'0" 

7'0"-8'0" 

TABLE  SHOWING  DISTANCES  APART  FOR  DIE  SQUARE  TIMBER  FOR  FALSE-WORK 


Scantling 

5"  x  5" 

6"  X  6" 

7"  X  7" 

8"  X  8" 

9"  x  9" 

Distances 

4'0" 

4'6"-5'0" 

5'6"-6'0" 

6'0"-8'0" 

8'0"-10'0" 

ORDER  OF  STRIKING  THE  FORMS 

The  order  of  striking  forms  is  thus  sum- 
marised by  Prof.  Johnson  :  (1)  Wedges  in 
column  form  struck  and  placed  in  sacks  for 
hoisting  to  next  floor  ;  (2)  clearance  pieces  ; 

(3)  sides    of    column    form    under    girder ; 

(4)  keys  at  beam  sides  ;  (5)  sides  of  column 
form  under  beam  ;  (6)  post  up  beam  as  close 
to  girder  as  possible  with  new  post ;  (7)  re- 
move posts  under  girder  bridge  ;  (8)  remove 


their  tops,  as  shown  in  Figs.  203  and  204 ; 
and  on  these  horses  the  significant  lines  are 
accurately  marked  with  a  knife.  For  a 
building  with  square  angles  (see  Fig.  203), 
one  main  line  having  been  determined  and 
marked  on  horses,  a  line  is  set  out  at  right 
angles,  by  means  of  a  builder's  square,  on 
another  row  of  horses,  and  the  operation  is 
repeated  until  the  necessary  rectangle  is 
formed.  Carefully  check  the  dimensions 


FORMS    AND    CENTERINGS 


157 


from  those  figured  in  the  drawings,  and  do 
not  trust  to  scaled  dimensions.  See  that 
opposite  sides  agree  in  length,  and  check  the 
accuracy  of  the  right  angles  by  measuring 


Fig.  204.— 

Horse  used  in 

Setting  Out 


lines,  the  lines  of  foundations,  etc.,  may  now 
be  marked  as  in  Fig.  203.  Acute  and  obtuse 
angles  are  set  out  as  in  Figs.  205  and  206  ; 
in  such  cases  the  accuracy  of  the  diagonal 
dimension  is  of  the  utmost  importance. 

FORMS  FOR  PILES 

For  a  square  pile  the  mould  required 
is  very  simple  ;  it  is  constructed  with  three 
sides  as  in  Fig.  207,  the  uppermost  side 
being  left  open  for  inserting  the  steel  skeleton 
and  concrete.  Angle  fillets  should  be  fitted. 
The  mould  must  lie  perfectly  level,  and  the 
skeleton  must  be  securely  suspended  in  the 
correct  position  and  be  dead  level,  so  that 


Fig.  203.— Setting  out 
Piers  of  Rectangular 
Building 


Fig.  206.  —  Setting 
out  Obtuse  Angle 
of  Building 

30  in.  one  way  and  40  in.  the  other  way,  the 
diagonal  between  the  two  points  then  meas- 
uring 50  in.,  which  equals  the  square  root  of 
the  square  of  30  plus  that  of  40.  The  outside 
lines  of  the  walls  and  columns,  the  centre 


Fig.  205.— Setting 
out  Acute  Angle 
of  Building 


it  may  be  entirely  surrounded  by  concrete. 
The  ends  of  the  rods  must  be  properly  placed 
in  the  cast-iron  shoe  at  the  end  of  the  mould. 
An  octagonal  pile  is  constructed  as 
above  described,  with  the  exception  that 


158 


REINFORCED    CONCRETE 


large  fillets  are  fixed  to  the  inside  of  the 
boxes. 

For  a  round  pile  a  mould  is  constructed 
with  two  circular  sides  or  cheeks,  made  with 
narrow  battens  after  the  fashion  of  a  barrel 
and  bound  with  iron  straps  ;  the  mould  must 
give  the  pile  two  flat  sides,  so  that  they  can 
lie  flat  up  against  the  guides  of  the  piling 
frame.  In  forming  this  mould  (see  Fig.  208) 
a  straight  plank  is  used,  and  through  it 
bolts  are  passed  about  2  ft.  apart  for  the 
purpose  of  holding  down  the  straps  that 
bind  the  circular  sides.  This  plank  is  laid 
down  dead  level  on  sleepers.  The  skeleton 
frame — the  reinforcement — is  next  laid  on 
this  plank,  and  then  the  circular  sides  or 
cheeks  are  attached  and  the  straps  bolted 
at  the  top,  as  shown.  The  open  space 

JfAYS  A&OUT  2-4 "  PfTCH 

•4- . 


joists  and  rebated  flooring  on  which  to  make 
and  cure  a  few  piles.  Figs.  210  and  211  show 
the  method.  The  side  shuttering  of  the 


I 


Fig.  208.— Bolted  Form  for  Round  Pile 

piles  is  formed  of  2-in.  by  8-in.  verticals  held 
together  by  1-in.  by  7-in.  cleats  bid  on 
rebated  flooring.  The  sides  of  the  shutter- 
ing are  kept  in  position  by  2-in.  by  4-in. 
distance  pieces,  the  whole  being  kept  in 
position  by  struts  and  1-in.  by  7-in.  boards 
temporarily  tacked  to  the  top. 

A  method  of  American  origin  is   shown 

in  Figs.  212  to  214  ;  the  parts  dissociate  as 

shown,  and,  for  curing,  the  green  piles  are 

placed  on  2-in.  by  12-in. 

)  planks  resting  on  beams 

— ' ' — >  3  ft.  6  in.  apart. 


FOUNDATION  FORMS 


Fig.   207.— Form  for  Square  Pile 


at  the  top  of  the  mould  is  left  for  the  con- 
creting, and  is  the  same  width  as  the 
bottom  flat  plank. 

The  same  shape  can  be 
made  by  using  wood  tem- 
plates of  the  shape  shown 
in  Fig.  209,  and  fixing  the 
narrow  battens  to  them. 
The  two  sides  are  held 
together  by  nailing  short 
stays  on  the  top  side,  as 
illustrated. 

As  in  the  case  of  a  square 
pile,  the  skeleton  frame- 
work is  suspended  by  hooks 
to  the  top  stays  to  keep  it 
in  correct  position,  and  the 
hooks  are  removed  when  the  concrete  is 
half-way  up  the  mould,  so  that  there  is  no 
chance  of  the  steel  framework  being  dis- 
turbed from  its  correct  position. 

Most   contractors   set   up   a   platform   of 


In  preparing  the  site 
for    a    foundation   slab, 
the  ground  must  be  brought  to  a  true  level 
and  boarding  placed  about  the  footing  if  the 
nature  of  the  soil  demands   it.     If   ribbed 

foundations  are 
STAYS  A&OVT  24  '  PfTCfr 


TEMPLATE 


used  they  must 
b  e  excavated 
for  and  suit- 
able boxing 
provided.  The 
concrete  is 
poured  into  the 


Fig.  209.— Stayed  Form  for  Round  Pile 


forms  in  layers  and  the  reinforcements  added 
at  the  proper  levels.  Fig.  215  shows  how  the 
foundation  forms  are  fixed  up  when  the  slab 
has  had  sufficient  time  to  set  hard  enough  to 
bear  the  weight  of  the  men  and  materials. 


FORMS    AND    CENTERINGS 


The  moulds  for  the  beams  are  very  readily  side  of  the  moulds  about  2  ft.  6  in.  apart, 
constructed,  it  being  only  necessary  to  form  as  shown  dotted  in  the  illustration.  They 
the  two  sides.  When  these  sides  are  fitted  up  can  be  withdrawn  when  the  sides  of  the 


hE-  - 
I4*1- 

r*:° 


- 

-  12.  CONCRETE. 

"" 

"vVALL 

Fig.  210 

/DISTANCE  PIECE  ^ 

FORM 


R3RM 


y  l/IOIHPH-L    rlCl^C  •  -^ 

~l" 

^                      >« 
PILE 

X                     xrf 

1 

M 

^                    •%; 
HLL 

1 

IE 

i 

^       ^ 

PILL 

ik             A 

sV 

ij           c{ 

{         4        if 

/       i!         tf         ij 

ZHZ  PLANki 


WALLPLATE 


z 


Fig.  211  (  FOLPINC  WEPGES 

Figs.  210  and  211.— Elevation  and  Enlarged  Cross  Section  of 
Pile-making  Platform 


•  -2*12.        t~2*a 

Fig.  212.— Plan  and  Ele- 
vation of  American 
Pile-making  Forms 
and  Platform 


Fig.   213.— Enlarged  Cross  Section  through 
Form  shown  in  Fig.  212 


Fig.  214.— Concrete  Piles 
and  Forms  Dissociated 


in  their  proper  positions  they  must  be  firmly 
stayed  and  strutted  to  each  other,  both  top 
and  bottom  as  shown.  By  carrying  the  top 
stays  over  the  width  of  the  end  beams  and 
strutting  the  off-side  of  these  to  long  pegs 


Fig.   215. — Foundation  Form  Built  after  Slab  is  Hard 


driven  into  the  ground  (if  it  is  firm  enough 
to  hold  them),  the  mould  will  be  secured 
without  breaking  away.  If  this  method  of 
strutting  from  pegs  is  impossible,  the  moulds 
may  be  held  by  bolts  passing  through  each 


moulds  are  struck.    Fig.  216    is    a    photo- 
graph showing  forms  for  pile  caps,  etc. 

The  ordinary  box  mould  for  foundations 
is  simply  four  sections  of  the  required  depth, 
two  of  the  sides  extending  beyond  the 

others,  all  bat- 
tened  and 
bolted  together 
(see  Fig.  217). 
The  2-in.  by 
2-in.  arris  fillet 
and  the  4-in. 
by  2-in.  batten 
on  the  ex- 
tended side 
form  a  groove 
for  the  bat- 
tened section 
to  slide  into.  The  4-in.  by  l|-in.  battens 
are  a  4-in.  by  3-in.  batten  once  sawn.  The 
square-headed  f -in.  rods  have  a  minus  thread, 
and  6-in.  by  6-in.  by  J-in.  square  plates  act 
as  washers.  By  alternating  the  fixed  square 


Fig.  217. — Typical  Box  Form  for  Foundation 


Fig.  216. — Forms  for  Pile  Caps,  Foundations,  etc. 


1 60 


FORMS    AND    CENTERINGS 


161 


head    of  rod   and  nut  the  form  is   made 
capable    of    easy   adjustment   to    bring    it 
square   and   can   be   readily  removed   and 
altered. 
In  the  type  of  form  shown  by  Fig.  218, 


by  4-in.  battens  to  serve  as  stop  pieces. 
The  four  sides  having  been  placed  in  posi- 
tion on  the  rough  base  and  nailed  together 
through  the  stop  pieces,  4-in.  by  4-in.  posts 
were  skew-nailed  on  the  outside  as  illus- 
trated, and  holes  bored  for  the  passage  of 
annealed  iron  wires,  which  connected  two 
facing  posts,  exactly  as  shown,  and  which 
were  tightened  by  twisting  with  a  rod.  The 
external  wires  were  cut  when 
6>"* fa' &OAKOS  the  time  came  to  remove 
the  forms,  the  projecting 
ends  being  hammered  in  and 
finished  ofi  with 
3*2  &KACL  a  trowel.  It  is 
advisable  to  intro- 
duce strainers  to 
take  the  stress 
caused  by  tighten- 
ing up  the  wires, 
and  to  prevent  the 
form  being  pulled 
out  of  shape. 


Fig.  218.— Cheaper  Type 
of  Form  for  Founda- 
tions 


the  3-in.  by  2-in.  braces  keep  the  mould 
rectangular  and  also  serve  as  handles.     This 
is    a    cheap    and    effective    type    of 
form  which  can  be  recommended  for 
repetition  work. 

Form  Strengthened  by  Twisted 
Wire. — In  the  United  States,  found- 
ation and  other  forms  are  often  held 
together  by  means  of  twisted  annealed 
wire,  all  bolted  rods  and  shoring 
being  dispensed  with.  While  English 
engineers  might  fear  that  die-square 
results  could  not  be  obtained  by  such 
a  method,  it  must  be  admitted  that 
it  is  cheap,  simple,  adaptable  to  a 
wide  range  of  applications,  and  de- 
serves to  be  more  extensively  em- 
ployed. Fig.  219  shows  such  a 
form,  the  detail  at  the  foot  of  the 
figure  illustrating  the  type  of  found- 
ation— square  tapering  piers  arched 
together — for  which  the  particular 
form  shown  was  required.  The 
sides  consisted  of  1-in.  boarding 
nailed  to  2-in.  by  4-in.  battens. 
Two  opposite  sides  were  14  in.  wider 
than  the  pier,  and  to  the  edges  of 
their  inner  sides  were  spiked  2-in 
11 


Fig.  219.— Form 
Strengthened 
witb  Wire 


Fig.  220. — Typical  Column 
Form  with  SHd-in  Front 
Boards 


162 


FORMS    AND    CENTERINGS 


163 


COLUMN,  BEAM,  AND  SLAB  FORMS 

The  most  frequently  used  forms  in  rein- 
forced concrete  are  those  for  columns,  beams 
and  floor  slabs,  so  that  any  economy  effected 


whole  fomfsides  at  once  with  vertical  board- 
ing held  in  position  by  battens,  this  method 
necessitating  tha  >pouring  in  of  the  concrete 
from  the  top,  and  consequently  the  use  of 


Fig.  221. — Typical  Column  Form  with  Spiked-on  Front  Boards 


in  their  design  enabling  their  re-use  without 
waste  is  sure  to  tell  favourably  in  the  cost 
of  the  structure. 

Three  Methods  of  Constructing 
Column  Forms. — There  are  three  methods 
of  setting  up  column  forms. 

The  first  is  to  construct  the  forms  for  the 


a    wet    mixture,    because   ramming    would 
probably  displace  the  reinforcements. 

The  second  method  is  to  build  up  the 
whole  four  sides  gradually  with  horizontal 
boards  held  in  position  by  battens  at  the 
angles,  and  brought  up  in  short  sections  as 
the  work  proceeds. 


/i 


&&OCT 


Fig.  222. — Clamped 
Form  for  Short 
Columns 


164 


Fig.  223.— Cheap  Type 
of  Column  Form 


165 


i66 


REINFORCED    CONCRETE 


The  third  method  is  the  most  economical, 
and  therefore  the  one  most  usually  adopted. 
It  consists  in  erecting  only  three  sides  in 
position  at  first,  formed  of  vertical  boards  ; 
the  fourth  side  is  gradually  brought  up  with 


as  the  work  proceeds,  in  short  lengths  placed 
between  a  guide  and  the  angle  fillet  on  each 
side.  The  other  portion  of  the  mould  is 
made  with  the  boards  vertically  battened 
and  distance-pieced  and  bolted  into  position. 


&QAGDS 


3*2 


Fig.  224. — Column  Form  with  Two  Sides  held  between  Fillets  and  Battens 


horizontal  boards  as  the  concrete  is  applied, 
thus  permitting  thorough  supervision  of  the 
work  as  it  progresses. 

The  edges  of  the  columns  are  usually 
chamfered  by  placing  small  angular  slips 
in  the  corners  of  the  forms. 

Fig.  220  shows  a  form  constructed  with 
three  sides  only,  the  fourth  being  built  up 


In  Fig.  221,  the  form  is  built  up  in  board- 
ing, 9  in.  by'^lj  in.,  all  horizontal,  strongly 
nailed  to  4-in.  by  2-in.  uprights.  To  make 
the  front  A,  the  boards  are  spiked  on  as  the 
work  proceeds.  The  back  is  held  together 
with  3-in.  by  2-in.' battens  to  allow  of 
quicker  re-use.  The  angle  fillets  are  lightly 
spiked  in  place. 


FORMS    AND    CENTERINGS 


167 


In  the  type  shown  by  Fig.  222,  the 
boarding  is  vertical  on  all  four  sides  and 
held  together  with  4-in.  by  l|-in.  battens 
strongly  spiked  on.  In  addition  to  the 
ordinary  spiking,  wooden  clamps,  out  of 
3-in.  by  2-in.  stuff,  and  bolted  together  at 
the  angles,  keep  the  form  in  shape ;  they 
rest  on  the  horizontal  battens.  This  method 
of  construction  is  employed  only  for  the 
shorter  height  of  pillars,|_and  has  the  dis- 


arranged at  intervals  according  to  the 
height  of  the  pillar.  The  life  of  this  type  of 
form  is  not  long,  because  of  the  spiking  on 
of  the  front  boards. 

In  the  method  shown  by  Fig.  224,  the 
9-in.  by  If-in.  boards  are  all  horizontal  and 
are  battened  together  on  sides  A,  B,  and  c. 
Side  D  is  inserted  as  the  work  rises,  as  short 
lengths  of  If-in.  board.  In  addition  to 
spikes,  the  form  is  bolted  through,  as  shown, 


Fig  225. — Column  Form  with  Two  Sides  held  between  Fillets  and  Battens 


advantage  .of  not  allowing  easy  inspection 
or  close  punning,  and  of  causing  derange- 
ment of  the  reinforcement  in  the  process  of 
depositing  the  concrete  in  the  mould. 
Spikes,  or  nails,  clenched  at  the  angles,  are 
sometimes  used  instead  "of  -bolts. 

Fig.  223  illustrates  a  cheap  type  of  form  in 
which  the  boards,  placed  on  as  the  work 
proceeds,  are  simply  spiked  or  strongly 
nailed  on  to  the  built-up  three  sides ;  6-in. 
by  IJ-in.  and  8-in.  by  IJ-in.  boards  are 
held  together'with  4-in.  by  2-in.  battens  on 
each  of  the  three  sides,  with  £-in.  bolt  rods 


and  sometimes  a  strainer  is  inserted  to  take 
the  pressure  of  the  bolts. 

Fig.  225  shows  a  case  in  which  6-in.  by 
2-in.  vertical  planks  are  battened  together 
on  three  sides  of  theTJorm  and  upright 
members,r3  in.  by  2  in.,  are  spiked  on  the 
sides  so  as  to  make  with  the  angle  fillets 
two  sets  of  grooves  to  receive  the  back  and 
front,  the  latter  being  slipped  in  from  the 
top  as  the  work  proceeds.  As  illustrated, 
the  batten  on  the  back  is  clear  of  the  bolt. 

The  forms  for  beams  are  made  in  a 
number  of  ways,  the  best  being  to  have  two 


i68 


REINFORCED    CONCRETE 


sides  and  a  base  easily  detachable  (see  Fig. 
226).  The  7-in.  by  2-in.  planks  are  joined 
together  by  4-in.  by  2-in.  battens  to  form 
the  base,  while  9-in.  by  IJ-in.  planed  boards, 
with  4-in.  by  2-in.  battens  form  the  sides. 


with  2-in.  planks  battened  together  with 
4-in.  by  2-in.  stuff,  the  battens  to  the  base 
being  extended  and  strutted  as  shown  with 
3-in.  by  2-in.  struts.  A  half-round  mould 
is  inserted  in  the  angles  to  give  a  rounded 


Temporary  ties,  say  4£  in.  by  \\  in.,  are 
nailed  across  the  top  before  filling  in  and 
punning.  The  battening  is  arranged  at 
3  ft.  6  in.  apart.  An  effective  finish  to 
the'arris  of  the  beam  is  obtained  by  insert- 
ing"^ hollow  mould  (see  Fig.  228)  instead 
of  the  angle  fillet. 


Fig.  226.— Typical 
Beam  Form 


finish   to  the   beams.     This   is,  how- 
ever, a  wasteful   design,  and   is  not 
adjustable  to  further  uses.     Fig.  231 
shows   a  form  for  producing   splayed 
angles  between  pillar  and  beam. 
The  system  of   false-work  shown  by'Fig. 
230  illustrates  the  junction  of  two  beams, 
and    also   the    floor  centering  in -position. 
The  dead  shores  are  arranged  at  intervals. 
At  the  edges  the  IJ-in.  boarding  is  rounded 
off  as  shown  to  give  a  good  finish  to  the  angles 
of  floor  and  beam.    The  method  of  striking 


»r 


/77 


Fig.  227. -Boards  with  Splayed  Edges  to  allow  for  Expansion 


In  using  dry,  sound  timber,  a  provision 
to  allow  for  swelling  is  made  by  running 
the  stuff  through  a  mill  arranged  to  splay 
one  edge,  as  shown  in  Fig.  227.  It  is  advis- 
able not  to  run  it  to  a  feather  edge,  but  to 
allow,  say,  a  TVm-  fillet. 

Fig.  228  shows  a  beam  form  constructed 


the  floor  centering  is  to  withdraw  the  4-in. 
by  2-in.  batten,  which  is  put  in  in  conveni- 
ent sections ;  drop  slightly  the  9-in.  by 
3-in.  joists,  and  then  take  out  the  boarding. 
Immediately  support  the  floor  with  head 
and  cill  piece  for  a  further  pericd.  This 
design  is  open  to  many  objections :  splayed 


Fig.  228.— Beam  Form 
Strutted  from  Ex- 
tended Base  Battens 


J*0££  PtATL 


Fig.  229.— Folding  Wedges 
under  Dead  Shore 


169 


iyo 


REINFORCED    CONCRETE 


fillet  is  better  than  tlie  hollow  moulding ; 
the  side  boards  had  better  be  horizontal; 
and  the  joists  should  not  rest  on  the  beam 
sides  but  be  propped  independently,  so  that 
the  sides,  of  the  beam  can  be  removed  first. 
The  centering  for  floor  slabs  is  built  up 
with  a  series  of  deals  from  beam  to  beam, 
supported  by  head  pieces,  dead  shores,  and 
cills  on  the  lower  floor  at  frequent  intervals. 


wedges.  Fig.  229  shows  the  arrangement  at 
the  base  of  a  6-in.  by  6-in.  dead  shore,  but 
it  is  preferable  to  have  the  wedges  at  the 
top  of  the  upright. 

Various  Beam  and  Column  Forms 
Described. — Fig.  232  illustrates  the  simple 
Hennebique  type  of  beam  form.  A  is  the 
bottom  board,  2  in.  thick,  the  exact  width 
of  the  beam,  and  supported  by  posts  F. 


Fig.  230.— Form  for  Two 
Intersecting  Beams 


See  that  the  shoring  is  taken  to  a  firm 
bearing — that  is,  carried  right  through  all 
the  floors  as  in  Fig.  246  (p.  176) — not  only  to 
support  the  weight  of  the  floor  being  con- 
structed, but  to  ensure  that  damage  is  not 
done  to  the  floors  already  in  yet  still  in  a 
somewhat  green  condition.  The  system  of 
wedging  should  be  arranged  in  such  a  way 
that  it  can  be  eased  at  given  periods,  ensur- 
ing the  beam  taking  an  even  setting  and 
bearing.  The  cills,  and  often  the  heads, 
are  provided  with  folding,  hard-wood 


At  the  top  of  the  side  boards  B  are  nailed 
battens  or  cleats  to  carry  the  ends  of  the 
secondary  beams  D,  also  supported  by 
posts  F.  The  upper  boards  E  of  the  main 
beam  forms  are  then  placed  in  position, 
together  with  the  side  boards  H  of  the 
secondary  beam  form.  Finally,  the  lagging 
or  floor  slab  centering  is  placed  on  the 
top  of  the  side  boards  of  the  beams.  The 
whole  is  held  together  with  iron  clamps,  as 
shown  in  Fig.  233.  At  the  bottom  of  each 
strut  there  should  be  hard-wood  folding 


Fig.  232. — Two  Cross  Sections  through  Henne- 
bique  Floor  and  Plan   of  one  Bay 


R 


CL/MP 


Fig.  233.— Beam  Form  held  by  Clamp 


BOLT 


SCRBff 


Fig.  234. — Column   Form   with 
Bolts  and  Thumb-screws 


Fig.  235 


7*2 

UPRIGHT 


WEDGES 


Fig.  236 


Figs.   235  and  236. — Beam  Forms  used  in  Messrs.   Sainsbury's  Premises,   London 


172 


FORMS    AND    CENTERINGS 


173 


wedges,  useful  in  setting  and  striking  the  inserting  wedges  as  shown.  A  section 
moulds,  and  these  should  be  adjusted  until  through  the  beam  form  is  shown  in  Fig.  235. 
all  is  perfectly  firm.  IE  the  filling  in  of  the  The  point  to  be  specially  noted  is  the  rebated 


Fig.   238.— Plan  of  Column 
Form 


SEDUCING 
FIUE.T- 


Fig.    237  — Beam    and    Column 

Forms    used    at    a  Bermondsey 

Warehouse 


Fig  239.— Plan  Showing  Posi- 
tion of  Clamps  for  Reduced 
Column  Form 

concrete  has  taken  the  camber,  previously 
allowed,  out  of  the  beam,  the  wedges  should 
at  once  be  driven  home,  so  that  the  camber 
is  again  obtained. 

The  beam  and  column  forms  adopted  at 
Messrs.  Sainsbury's  pre- 
mises in  Stamford-  Street, 
London,  S.E.,  are  shown 
in  Pigs.  234  to  236.  The 
column  forms  of  1^-in.  ver- 
tical planed  boards  were 
built  up  on  three  sides 
complete.  A  shutter,  made 
in  two  heights,  of  IJ-in. 
boards  with  cross  battens 
was  used  for  the  fourth 
side.  The  whole  was 
clamped  together  by  2-in. 
by  5-in.  clamps  1  ft.  6  in. 
apart,  adjusted  at  the  corners  with  bolts 
and  thumb-screws ;  the  angles  of  the 
columns  were  ovolo  moulded  by  the  insertion 
of  corner  fillets  as  in  Fig.  234.  The  columns 
can  be  reduced  inside  the  same  clamps  by 


support  to  the  beam  bottom.  By  means  of 
fillets  of  various  sizes  placed  in  these  rebates, 
the  beam  can  be  made  of  less  breadth  or 
depth,  and  so  save  cutting  up  the  cheeks  of 
the  beam  mould.  Fig.  236  shows  how  the 


Fig.    240.— Isometric 

Sketch  of  Forms  for 

a  complete  Bay  of  a 

Warehouse  Floor 


beam  form  may  be  reduced  in  depth  and 
width  without  destroying  any  material. 

Figs.  237  and  239  show  the  beam  and 
column  forms  used  at  Peak,  Frean  and 
Co.'s  warehouse  in  Bermondsey.  The  beam 


174 


Fig.  242. — Beam  and  Column  Forms  used  at  H.M.'s  Stationery  Office 


IB. 


D//////y/ 


:i  J^'BOO- 

-< 

«—  i"  o"  —  * 

/ 

/ 

<p 

/ 

S>'*3* 

We. 

-f 

71 

m 

B--I 

.  .  .  .  . 

_ 

~ 

-•:  _ 

m 

t- 

ttft- 

t  

^p 

*v 

Kf- 

*      P05T3 


Fig.  243  Fig.  245  . 

Figs.  243  to  245.— Adjustable  Beam  Forms  designed  by  H.  Kempton  Dyson 


175 


176 


FORMS  AND  CENTERINGS 


177 


forms  are  supported  by  2-in.  by  4-in.  uprights 
braced  by  cross  battens  nailed  on.  The  top 
cross  brace  at  the  under-side  of  the  beam 
is  bolted  to  the  uprights,  and  it  supports 
the  bottom  board  of  the  beams.  With 
reference  to  the  column  forms,  should  it  be 
desired  to  reduce  the  size  of  the  column, 
the  lower  pair  of  clamps  is  brought  closer 
together  and  the  upper  pair  fitted  to 
the  grooves  in  the  lower  pair  and  wedged 
in  the  new  position.  The  column  can  be 
reduced  in  the  other  direction  by  inserting 


Fig.  247.— Form  for  Twelve-sided  Column 


3/&  DOLTS 


FLUTE   nOULDS 
OF  PLASTER  OF  PAfUS 


Fig.  248.— Form  for  Fluted  Column 

fillets  of  the  proper  size  between  the  clamps 
and  the  boarding  as  shown.  The  main  beam 
forms  are  supported  on  2-in.  by  4-in.  up- 
rights in  pairs  at  every  3  ft.  6  in.  of  the 
length.  The  top  cross  pieces  immediately 
under  and  supporting  the  bottom  board  of 
the  beam  are  bolted  to  the  uprights.  The 
secondary  beams  rest  on  fillets  nailed  to  the 
sides  of  the  main  beams  and  also  on  2-in. 
by  4-in.  uprights  at  3-ft.  6-in.  centres.  The 
lagging  of  the  floor  slab  is  supported  on  2-in. 
by  4-in.  joists  2  ft.  apart,  the  ends  of  the 
joists  resting  on  the  secondary  beam  forms. 
12 


Fig.  240  gives  a  good  idea  of  the  usual 
construction  of  the  centering  of  floor  slab, 
girders  and  secondary  beams ;  the  illus- 
tration shows  a  complete  bay  of  a  warehouse 
floor  in  isometric  projection. 

Beam  and  Column  Forms  in  H.M.'s 
Stationery  Office. — Fig.  242  shows  the 
beam  and  column  forms  for  the  Government 
Stationery  Office  in  Stamford  Street,  S.E. 


i 
i 

.  — 

TTTTI  ' 

(    1  1  1  1  U 

-t 

ui'11  '  * 

.     " 

Fig.   249. — Form  for  Diminished  Column 

The  main  beams  have  a  span  of  21  ft.  3£  in., 
and  the  intermediate  beams  15  ft  2  in.  The 
columns  are  20  in.  by  20  in.  There  are  so 
many  bays  of  exactly  similar  dimensions  that 
the  centering  could  be  used  over  and  over 
again.  The  drawings  explain  themselves, 
but  a  point  to  be  noted  is  the  method  of 
supporting  the  boarding  for  splaying  the 
haunches  of  the  beam  on  2-in.  by  4-in. 
fillets  nailed  to  the  column.  A  photograph 
showing  much  of  the  false- work  is  reproduced 
in  Fig.  241 ;  the  steel  gantry,  which  is  a 
conspicuous  feature  in  this  view^  was 


•3 
33 
c 

I 


U 


178 


179 


i8o 


REINFORCED    CONCRETE 


employed  for  raising  the  building 
materials,  electric  travelling  cranes 
running  upon  it. 

Beam  Forms  Designed  by  H. 
Kempton  Dyson.— Figs.  243  to  245 
show  beam  forms  designed  by  H. 
Kempton  Dyson.  A  point  to  be 
noted  is  that  the  beam  can  be  re- 
duced in  depth  by  means  of  a 
blocking  piece.  The  cleats  on  the 
uprights  can  be  readjusted  to  any 
height,  thus  saving  the  cutting  of 
the  uprights  to  different  lengths 
to  vary  the  height.  A  different 
size  of  base-plate  at  the  bottom  of 
the  uprights  may  also  be  employed 


Fig.  253. — Form  for  Column  Base  at  Wesleyan  Hall,  Westminster 


Fig.  254.— Centering  for 
Floor  at  Wesleyan 
Hall,  Westminster 

for  the  same  purpose. 
The  cross  bearers  under 
the  beam  bottom  support 
2-in.  by  5-in.  joists,  which 
in  turn  support  the  2-in. 
by  4-in.  joists  carrying 
the  floor  slab  boards. 
These  joists  as  arranged 
are  not  the  full  length: 
between  the  beams,  and 
wedges  are  inserted  be- 
tween their  ends  and  the 
beam  boxes  to  keep  the 
latter  in  position.  The 
boarding  carrying  the 
slab  runs  parallel  to  the 
beams,  and  can  be  used 
in  fairly  long  lengths, 
thus  saving  waste  in 
cutting.  The  joists  being 
supported  direct  from 
the  uprights,  the  parts 


FORMS    AND    CENTERINGS 


of  the   beams,   floor,  and   columns  can  be 
easily  struck  independently  of  each  other. 
Various  Column  Forms. — The  form  for 


of  cement.  Figs.  250  to  254  are  photo- 
graphic views  showing  the  false-work  to 
beams  and  columns. 


Fig.   255. — Centering  Resting  on  Flanges 
of  Steel  Joists 


a  twelve-sided  column,  illustrated  by  Fig.  247> 
is  made  of  wooden  staves  dowelled  together 
and  hooped  with  adjustable  iron  straps. 
The  form  for  a  fluted  column,  shown  in 
Fig.  248,  consists  of  narrow  laggings  dowelled 
together  and  hooped  round  with  adjustable 


Fig.   256. — Centering  Suspended  from 
Flanges  of  Steel  Joists 

iron    bands.      The  flutes    are    formed    of 
plaster-of-paris   attached  to  the  inside   by 
screws  which  are  inserted  from 
the  outside  ;  in  removing  them, 
the  screws  are   drawn   and  the 
plaster  flutes  remain  as  a  pro- 
tection after  striking  until  the 
building  is  completed. 

In  the  new  Wesleyan  Hall  at 
Westminster  circular  columns 
with  entasis  are  used.  The 
forms  (Fig.  249)  were  made  in 
sections  4  ft.  high,  the  narrow 
vertical  laggings  being  easily  bent  to  the 
required  curve  and  secured  to  horizontal 


2  PLANK,  2 8  CENTRES 


Fig.   258.— Centering  for  Arch  Ceiling 
between  Joist  Flanges 

CENTERINGS    FOR    CONCRETE    FLOORS 
CARRIED  BY  STEEL  JOISTS 

The  simpler  varieties  of  centering  for 
steel  joists  will  now  be  briefly  dealt  with. 
Fig.  .255  shows  a  concrete  slab  lying  on 
the  top  flanges  of  rolled  steel  joists ; 
timber  bearers  rest  on  the  bottom  flanges 
and  support  the  lagging.  Fig.  256  shows  a 
similar  centering  supported  on  the  bottom 
flanges  by  means  of ,  hook  hangers  or  sus- 
pended by  sb'ngs  from  the  top.  It  may  be 
arched  as  shown  by  dotted  lines.  Fig.  257 
shows  a  design  by  W.  F.  Kearns  for  a  rein- 
forced concrete  floor  with  steel  main  beams. 
The  defect  of  these  three  floors  is  that,  even 


-  2-0  CENTRES 
Figs.  259  and  260. — Floor  Centering  supported  by  Hangers 


2*8  WOOD  W4SHERS 
ffe  BOLTS    HAMD  NUT 

Fig.   257. — Centering   for  Concrete  Floor 
having  Steel   Main  Beams 

pieces  set  out  with  their  outer  edges 
plumb,  while  their  inner  edges  coincided 
with  the  diameter  of  the  column  ;  these 
horizontal  pieces  were  fixed  at  intervals  of 
2  ft.  The  method  gave  such  good  results 
that  the  columns  could  be  finished  with  in. 


although  the  centering  is  kept  low,  the 
bottom  flanges  of  the  steel  joists  are  not 
embedded  in  the  concrete,  and  therefore  are 
exposed  to  the  effects  of  fire  ;  the  defect  is 
obviated  by  having  a  ceiling  on  iron  hangers, 
metal  lathing  being  embedded  in  the  plaster. 
Fig.  258  shows  the  centering  for  an  arch 
ceiling  between  the  joist  flanges,  suspended 
by  f-in.  bolts.  In  the  method  shown  by 
Figs.  259  and  260,  square-headed  hooked 
bars  are  hung  on  runners  which  are  placed 
on  the  upper  flanges  of  the  steel  joists, 
the  bars  being  placed  alternately  on  each 
side  of  the  joist. 

WALL  AND  PARTITION  FORMS 

The  shuttering  for  walls  consists  of  bat- 
tened widths  passed  down  behind  quarter- 
ing driven  into  the  ground  and  kept  apart 
by  iron  distance  pieces  at  the  top,  the 


i8z 


REINFORCED    CONCRETE 


arrangement  for  battering  the  face  being  on  it,  to  which  the  uprights,  spaced  at  not 
obtained  by  sloping  the  uprights  to  the  more  than  5-ft.  centres,  are  fixed  and  braced 
required  -angle.  In  the  case  shown  by  by  runners  and  struts  to  keep  the  horizontal 

boards   as  rigid   as   possible.     The   outside 
shuttering  is  carried  up  to  the  full  height 


Fig.  262.— Section  of 
Wall   Form 


Fig.  261.  — Form  for 
Wall.  Part  is  Raised 
to  Second  Position 


Fig.  261  shutters  are  made  of  7-in.  by  2-in. 
planks  with  4-in.  by  2-in.  ledgers,  and  by 
employing  sleeve  pieces  over  the  f-in.  rod 
bolts  fitted  with  heads,  nuts,  washers,  etc., 
the  sides  of  the  form  are  kept  the  required 
distance  apart.  In  the  right-hand  part  of 
the  illustration  a  section  of  the  wall  is  shown 
complete,  and  the  shutter  has  been  raised  to 
the  second  position  resting  on  the  sleeve 
pieces,  as  further  shown  in  Fig.  262.  In  the 
next  illustration  (Fig.  263)  an  alternative, 
but  inferior,  method  is  shown,  the  6-in.  by 
l£-in.  boards  having  4-in.  by  2-in.  continuous 
head  and  cill  pieces. 

In  the  type  of  form  illustrated  in  Figs. 
264  and  2(>r>  it  will  be  seen  that  the*  floor  slab 
having  first  been  laid,  cill  pieces  are  placed 


Fig.   263.— Panel  for  Wall  Form 


FORMS    AND    CENTERINGS 


183 


the  inner  shuttering  is  made  in  panels  5  ft. 
wide  and  2  ft.  6  in.  high,  and  built  up  as  the 
work  proceeds.  The  size  of  the  uprights  is 
governed  by  their  distance  apart,  the  height 
and  thickness  of  wall,  and  amount  of  strut- 
ting. This  design  is  not  recommended. 
For  walls  from  6  ft.  to  14  ft.  high,  2-in. 


is  used  the  studding  can  be  placed  farther 
apart,  and  of  course  made  correspondingly 
heavier  in  scantling. 

Fig.  266  shows  the  false-work  for  a  simple 
foundation  wall.  First,  2-in.  by  6-in.  hori- 
zontal bearers  are  laid  to  2-in.  by  4-in.  up- 
rights, the  latter  having  pointed  ends  which 


5™^Z 

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K'   6" 

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snot*-- 

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3TUD 


Figs.  264  and 
265.— Plan  and 
Cross  Section 
of  Shored-up 
Form  for  Wall 


by  7-in.  or  3-in.  by  6-in.  uprights  may  be 
used  if  not  placed  more  than  2  ft.  6  in.  apart 
or  2  in.  by  5  in.  if  spaced  not  more  than  2  ft. 
apart.  The  uprights  should  be  stayed  by 
one  or  two  rows  of  horizontal  runners 
2  in.  by  7  in.,  or  flooring  boards  propped 
up  by  2-in.  by  4-in.,  2-in.  by  5-in.,  or  3-in. 
by  4-in.  struts,  whichever  scantling  is  at 
hand.  It  follows  that  if  2-in.  shuttering 


are  driven  into  the  soil ;  the  bearers  are 
kept  in  position  by  being  secured  to  stakes 
driven  into  the  ground.  The  uprights  are 
braced  and  kept  the  proper  distance  apart 
by  1-in.  by  4-in.  battens  nailed  to  their  tops, 
vertical  packing  pieces  |  in.  square  being 
placed  between  the  studs  to  stay  the  shut- 
tering ;  after  a  section  of  the  concrete  has 
been  finished,  the  boards  are  raised  to 


184 


REINFORCED    CONCRETE 


continue    the   wall  above.     The  footing  is 
formed  by  allowing  the  first  layer  of  concrete 
to  flow  under  the  bottom  boards. 
Ransome's  Wall  Form. — Fig.  267  shows 


Fig.   266. — Form  for  Simple  Foundation  Wall 


a  form  much  used  in  America,  and  patented 
by  E.  L.  Ransome  in  1885.  The  vertical 
standards  are  formed  of  two  1-in.  by  6-in. 
boards  on  edge  with  a  slot  between,  through 
which  pass  the  bolts  (see  Fig.  269).  By 
undoing  the  bolt  the  planks  behind  the 


joints  in  each  row  coincide.  Long  6-in.  by 
6-in.  timbers  are  next  placed  vertically  at 
the  joints,  just  fitting  in  the  spaces  left 
between,  the  short  6-in.  by  6-in.  ledges, 
being  3  in.  from  the  ends  of  the  form.  Other 
rows  are  added  similarly  to  suit  the  height 
of  the  wall.  These  long  6-in.  by  6-in. 
verticals  are  kept  in  position  by  6-in.  by 
6-in.  horizontal  wales  placed  about  2  ft.  6  in. 
apart. 

This  set  of  operations  is  carried  out  on 
both  sides  of  the  wall ;  f-in.  diameter  rods, 
threaded  at  both  ends  and  fitted  with  nuts, 
are  passed  through  the  6-in.  by  6-in.  wales, 
clear  of  the  6-in.  by  6-in.  uprights,  through 
the  2-in.  planking  every  4  ft. ;  2-in.  by  4-in. 
distance  pieces,  the  exact  width  of  the 
wall,  are  placed  in  the  forms  quite  close 
to  the  bars,  and  these  distance  pieces  are 
knocked  out  as  the  concrete  reaches  them. 
After  the  removal  of  the  forms  the  pro- 
jecting ends  of  the  iron  rods  are  cut  off,  and 
covered  in  with  a  little  cement  put  over 
the  ends.  In  a  particular  instance,  this 
method  of  holding  the  forms  proved  very 
successful,  not  the  slightest  bulging  occurring 
although  the  wall  was  a  thick  one. 


Fig.   269.— Collar  and    Set-screw 
at  X  (Fig.  267) 


standards  can  be  set  free 
and  the  standards  raised 
as  the  wall  proceeds. 
The  walls  are  in  4-ft.  sec- 
tions. A  core  box  can 
be  introduced  for  hollow 
walls. 

The   construction  of    a 
large    panel    shutter    for 
walls  is  shown  by  Fig.  268. 
The  panels  are  made  of  2-in.  by  8-in.  plank 
dressed  one  side.     The  panels  are  erected 
by  placing  the  bottom  row  in  position  and 
fixing  them  by  stay  lasts.     Then  another 
row  is  placed  on  top  so  that  the  vertical 


:'s  Wall  Form 
/  f*Z  PLANKS 

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VERTICALS 

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Fig.  268.— Panel  Shutter 

IMITATING  MASONRY  WALLS 

A  method  adopted  by  E.  L.  Ransome  is 
to  fasten  wood  V  strips  to  the  wall  forms, 
so  as  to  produce  an  imitation  of  masonry 
(see  Fig.  270).  The  surface  may  be  left 


FORMS   AND    CENTERINGS 


185 


plain  or  rusticated  by  the  application  of  a 
pick  or  chisel. 

STAIRCASE  FORMS 

The  forms  on  which  the  stairs  at  Peek, 
Frean  and  Co.'s  warehouse  at  Bermondsey 


Fig.  270.— Side  of  Form  for  Imitating 
Masonry  Wall 

were  made  are  shown  by  Figs.  271  to  273. 
They  are  in  continuous  flights,  with  half- 
space  landings.  The  stringers  under  the 
soffit  of  the  stair  are  2  in.  by  6  in.,  and  the 
carriage  2  in.  by  4  in.,  with  cross  stemming, 


3  in.  by  6  in.  Horizontal  ledgers  2  in.  by 
5  in.  are  fixed  to  stay  the  uprights  and 
stringer.  The  soffit  of  the  steps  is  formed 
by  flooring  boards  being  nailed  to  the 
stringer.  The  form  for  the  steps  themselves 
is  constructed  of  triangular  bracket  pieces 
nailed  to  the  flooring  with  a  concave  moulded 
fillet  to  form  the  nosing  to  the  riser  (see  Fig. 
273). 

In  many  cases,  stairs  are  made  in  advance 
and  erected  similarly  to  stone  hanging  steps. 
In  that  case  the  stringer  is  moulded  like  an 
inclined  beam  properly  reinforced  top  and 
bottom  with  rods,  and  rebated  on  the  lower 
edge  to  receive  the  steps.  An  examination 
of  the  form  for  the  steps  will  show  at  a 
glance  that  the  usual  back  check  in  hanging 
steps  is  got  by  fixing  in  triangular  fillets. 

METAL  FORMS  FOR  WALLS,  ETC. 

A  recent  system  of  metal  forms  for  walls, 
columns,  and  girders  is  that  of  the  Blaw 
Steel  Centering  Co.,  and  it  is  designed  to 
combine  economically  the  three  processes 


Fig.  272.— Stringers 
and  Carriage  of  Stairs 


Fig.  271.— Form  and  Centering  for  Staircase 


Fig.  273. — Form  for 
Steps 


as  shown  in  the  isometric  sketch.  The 
stringers  are  supported  at  intervals  by  _2-in. 
by  5-in.  uprights ;  where  the  uprights 
require  to  be  more  widely  spaced,  they  are 


of  form  erection,  namely,  the  assembling  of 
forms,  lining  them,  and  spacing  them.  The 
panels  shown  in  Fig.  274,  which  make  up 
the  wall  forms,  are  based  on  a  standard 


i86 


REINFORCED    CONCRETE 


sized  surface  of  24  in.  square.    Forms  are      Corner  panels  are  supplied  with  returns  12  in. 
furnished  in  fractional  sizes  25  in.  by  12  in.,      on  each  side  and  24=  in.  high.     The  panels 


UHER3 
to  IO  APART 


Fig.  274.— Metal  Panel  Form  for  Walls 


Fig.  278.— Form  for  Spandrel 
Wall  to  Bridge 


Fig.  275.— Method  of 
Fastening  Panel 
Flanges  together 


Fig.  276.— Detail  of 
Metal  Beam  Form 


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Fig.   277. — Ransome  Form  for  Cornice 


Fig.   279.— Form  for  Curtain  Wall  with 
Moulded  Cornice 


24  in.  by  6  in.,  12  in.  square  and  6  in.  square,  have  angle  flanges  on  all  four  sides,  these 
These  sizes  enable  the  contractor  to  work  to  being  spot  welded  to  sheet  steel  plates.  There 
any  dimensions  which  are  a  multiple  of  6  in.  are  no  rivet  marks  on  the  face  of  the  forms. 


FORMS '  AND    CENTERINGS 


187 


The  plates  are  joined  by  special  fastenings 
passing  through  slots  in  the  flanges.  The 
flanges  are  fastened  together  as  shown  in 
Fig.  275.  Adjustable  beam  and  girder 
moulds  are  also  made,  using  wooden  planks 


'    "  Rv 

r~^£zlh 


wfl!. 


^Bti 


Fig.  280.— Two    Sections  of  Form    for 
Cornice  to  Hollow  Coping 

as  the  bottoms.  These  pieces  are  so  clamped 
together  that  the  sides  may  be  removed  and 
used  in  another  part  of  the  work,  while  the 
bottom  board  form  remains  in  place  securely 
shored  until  the  curing  of  the  concrete  is 
complete  (see  Fig.  276). 


HINGED 
Fig.  281. — Form  for  Ornamental  Parapet 

FORMS     FOR     CORNICES,     MOULDINGS, 
ETC. 

Fig.  277  shows  Ransome  forms  applied 
to  a  cornice.  The  false-work  for  a  spandrel 
wall  to  a  bridge  is  shown  by  Fig.  278.  The 
shuttering  is  1|  in.  thick,  kept  in  position 
by  2-in.  by  4-in.  studs  with  block  pieces. 
The  mouldings  are  made  by  inserting 
splayed  and  ovolo  or  cavette  moulded  slips 


in  the  angles  of  the  form.  A  detail  of  the 
form  for  a  curtain  wall  with  moulded  cornice 
and  frieze  is  illustrated  in  Fig.  279,  while 
Fig.  280  represents  forms  for  the  cornice  to 
a  hollow  coping. 


Cf#r- 


Abo 


Figs.  282  and  283.— Vertical  and  Horizontal 
Sections  of  Form  for  Battered  Retaining 
Wall 

FORMS  FOR  ORNAMENTAL  PARAPET 

Fig.  281  shows  the  form  for  an  ornamental 
parapet,  2  ft.  high,  to  a  bridge  or  balus- 


i88 


REINFORCED    CONCRETE 


trading.  Two  boards  7  in.  wide, 
corresponding  to  the  length  of 
the  panel,  one  each  for  the  top 
and  bottom,  lap  over  the  side 
board,  which  is  6  in.  wide.  Two 
4-in.  boards  are  used  for  one 
side  of  the  rail,  and  two  2J-in. 
boards  to  mould  the  inside  edge 
of  the  rail. 

For  the  panel  centre  two  8-in. 
boards  joined  together  are  nailed 
to  the  two  4-in.  boards.  The  two 
7-in.  top  and  bottom  boards  are 
hinged  to  this  form  so  as  to 
fold  back  to  allow  the  mould  to 
be  easily  emptied.  The  two  ends 
are  of  7-in.  timber,  hinged  to 
close  in  the  ends  of  the  forms. 

FORMS    FOR    RETAINING 
WALLS 

A  form  for  a  battered  re- 
taining wall  is  shown  in  vertical 
section  by  Fig.  282,  a  horizontal 
section  on  line  A  B  being  shown 
by  Fig.  283.  In  this,  4-in.  by 
3-in.  standards  are  driven  firmly 
into  the  ground  and  are  provided 
with  a  cross-head  and  4-in.  by 
3-in.  struts.  The  6-in.  by^3-in. 
planks  are  battened  together  and 
arranged  to  come  alongside  the 
4-in.  by  3-in.  standards.  Centre- 
bit  holes  are  made  through  the 
batten  and  standard,  and  f-in. 
bolts,  9  in.  long,  passed  through 
and  tightened  up. 

Other  forms  are  illustrated  by 
three  examples,  the  first  of 
which  is  a  retaining  wall  at 
Bridlington,  carried  out  by 
Ernest  Matthews,  A.M.I.C.E., 
and  clearly  illustrated  by  Figs. 
284  and  285.  The  height  of  the 
wall  varies  from  10  ft.  to  13  ft. 
with  buttresses  at  lOJt.  centres. 
The  cutting  is  excavated,  and 
IJ-in.  by  9-in.  rough  poling 
boards  are  placed  against  it  with 
two  rows  of  wales  in  the 
height.  These  are  shored  as 
shown  with  4-in.  by  4-in.  struts. 
The  foundation  slab  having  been 
put  in,  the  2-in.  by  5-in.  up- 
rights, at  about  2-ft.  centres, 
are  erected  on  the  foundation 
slab  and  spaced  apart  sufficiently 


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BUTTRESS 

1 

9"  l!4  POLING    BOARDS 


Figs.  284  and  285.— Plan  and  End  Elevation  of 
Form  and  Centering  for  Retaining  Wall  at 
Bridlington 


FORMS    AND    CENTERINGS 


to  allow  of  the  thickness  of  the  wall  and 
1£  in.  of  shuttering  on  each  side.  The 
uprights  are  held  in  position  by  two  rows 
of  1-in.  by  7-in.  flooring  nailed  on  and 
strutted  with  3-in.  by  4-in.  raking  and 
horizontal  struts  to  the  cutting  as  shown, 


with  vertical  flooring  boards  cut  to  the  rake 
and  nailed  to  1^-in.  by  5-in.  stops,  which 
are  secured  to  the  uprights.  The  forms  for 
the  buttress  should  be  boarded  in  on  top, 
ahead  of  the  concrete  laying,  with  1-in.  by 
7-in.  flooring  on  the  slope.  The  forms  are 
wetted  both  inside  and  out  before 
any  concrete  is  placed  in  them. 
A  2-in.  by  7-in.  board,  acting  as  a 
wind  strut,  is  nailed  to  the  top  of 
the  uprights  and  the  waling  at 
the  cutting. 

Retaining  Wall  at  Local 
Government  Offices. — The  forms 
for  the  retaining  wall  at  the 


Fig.  287.— Diagram,  Plan, 
and  Details  of  Centering 
for  Retaining  Wall  at 
Local  Government  Offices 


Fig.   286. — Part  End  Elevation  and  Section  of  Centering  for  Retaining 
at  Local  Government   Offices 


Wall 


the  shorter  ones  being  2  in.  by  4  in.  The 
top  and  bottom  portions  of  the  wall  should 
be  strutted  independently  of  each  other. 
The  uprights  being  securely  stayed,  the  first 
two  l|-in.  by  8-in.  boards  are  placed  on  the 
wall  side  of  the  uprights.  The  back  is  then 
proceeded  with  similarly. 
The  two  sides  of  the  buttresses  are  formed 


Local  Government  Offices,  carried  out 
by  the  Trussed  Concrete  Steel  Co.,  are 
shown  by  Figs.  286  and  287.  The  concrete 
at  the  basement  level  is  more  than  24  ft. 
below  the  street  level,  and  the  excavation 
was  done  in  five  sections  of  roughly  4  ft. 
each.  As  each  section  was  excavated,  1-in. 
by  7-in.  rough  vertical  sheeting  or  poling 


IQO 


REINFORCED    CONCRETE 


boards  were  placed  against  the  earth,  these 
being  secured  with  4-in.  by  9-in.  horizontal 
wales  and  2-in.  by  11-in.  vertical  planks 
and  raking  shores.  The  shores  were 
placed  11  ft.  or  12  ft.  apart.  The  top 
and  third  shores  were  formed  with 
4-in.  by  9-in.  timbers  bolted  together, 
the  intermediate  shores  being  7-in.  by 
7-in.  balks.  As  will  be  seen  from  the 
illustration,  the  thrust  of  the  earth 
against  the  retaining  wall  was  resisted 
by  trussed  and  braced  buttressed 
division  walls  with  intermediate  pilas- 
ters. 

When  the  earth  was  properly  held  up, 
the  forms  for  the  wall  were  begun. 
The  wall  was  5  in.  thick  with  9-in.  by 
9-in.  and  9-in.  by  12-in.  pilasters.  The 
face  of  the  wall  is  14  in.  from  the  cutting. 
3-in.  by  4-in.  uprights  were  fixed  up 
as  shown,  the  boards  being  1|  in.  thick 
and  fixed  to  the  uprights  a  few  boards 
at  a  time  as  the  concreting  proceeded. 
The  forms  for  the  pilasters  were  of 
If-in.  by  9-in.  vertical  flooring  boards, 
the  front  being  added  as  the  concrete 
proceeded.  The  column  and  strut  boxes 
were  framed  on  three  sides  with  IJ-in. 
by  9-in.  flooring  boards,  the  fourth 
being  added  in  advance  of  the  con- 
crete. The  boards  were  clamped  by 
1-in.  by  6-in.  battens  nailed  on  at 
2-ft.  centres.  As  the  wall  proceeded, 
the  poling  boards  and  wales  were  taken 
out  and  the  back  space  filled  in  with 
concrete.  Only  the  shores  were  left  in 
till  the  wall  was  finished,  and  when  they 
were  taken  out  the  holes  left  in  the 
wall  were  filled  up.  All  the  boxing 
was  of  IJ-in.  by  9-in.  boarding  with 
1-in.  by  6-in.  battens  ;  the  arrises  were 
taken  off  the  columns  and  beams  with 
angle  fillets. 

Another  Retaining  Wall.— Figs. 
288  and  289  illustrate  the  method 
adopted  in  carrying  out  the  deep 
basement  in  reinforced  concrete  at  the 
premises  of  the  British  and  Foreign 
Blind  Association,  Great  Portland 
Street,  London.  The  basement  has  an 
area  of  35  ft.  by  55  ft.,  and  is  31  ft. 


out  in  six  sections  of  5  ft.  deep.  Imme- 
diately the  first  section  was  excavated, 
1|  in.  upright  sheeting  was  laid  to  the  earth, 


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uu  ii/.  vy  uu   it.,  ana  is   01  it.     p. 
below  the  level  of  the  pavement.    The 
walls    on   three    sides    are    reinforced 
against   a   possible   24   ft.    head   of   water. 
The   fourth   side    being   an    existing   brick 
wall  is  deep  enough  to  render  underpinning 
unnecessary.     The   excavation   was   carried 


.  288  and  289.— Plan  and  Section  of  Centering, 
etc.,   for  Retaining  Wall  in  Deep  Basement 

waled  in  the  centre  with  10  in.  plank,  and 
strutted  right  across  the  basement  with 
five  balks  10£  in.  square  and  9  ft.  apart ; 
folding  wedges  were  used  to  tighten  up 


FORMS    AND    CENTERINGS 


191 


the  struts  against  the  brickwork  on  the 
fourth  side.  Three  cross  struts,  9  in.  by  9  in., 
were  used  to  stay  the  sheeting  on  the  end 
walls,  and  were  continued  as  stays  to  the 
large  lOJ-in.  balks,  thus  making  a  rigid 


] L 


Figs.  290  and  291.— Elevation  and  Plan  of  Form  and 
Centering  for  Silo 


rame.  The  balks  and  struts  had  to  be 
placed  so  as  not  to  hinder  the  carrying  out 
of  work  on  the  pillars.  The  next  section 
was  proceeded  with  similarly,  until  the 
excavation  was  carried  down  to  the  full  depth. 
The  wall  was  14  in.  at  the  bottom,  tapering 
in.  at  the  top.  Half  way  up  the  walls, 


on  the  outside,  large  strap  beams  were  intro- 
duced to  meet  the  thrust  of  the  earth  and 
water,  the  thrust  being  further  met  ,by 
introducing  horizontal  reinforced  concrete 
strut-beams  between  the  walls  and  the 
internal  pillars,  which  carry 
heavy  weights  from  the  super- 
structure. Horizontally,  the 
i  wall  was  strengthened  by 
the  addition  of  pilasters.  In 
order  that  the  wall  should 
be  monolithic  it  was  carried 
up  in  continuous  layers. 

The  walls  and  pilasters 
were  carried  up  as  described 
for  the  preceding  example, 
the  horizontal  boarding  of 
the  walls  being  secured  by 
upright  posts,  2  ft.  to  3  ft. 
apart,  the  outer  boarding 
being  added  as  the  work  pro- 
ceeded. The  pilaster  sides 
were  formed  by  l£-in.  ver- 
tical flooring,  the  front  of 
horizontal  boards  being  added 
as  the  work  proceeded.  As 
previously  described,  the 
poling  boards  and  walls  were 
taken  out  at  the  back  as 
the  work  proceeded,  and 
only  the  shores  were  left  in 
till  the  completion  of  the 
wall.  The  boxing  of  the 
strut  beams  and  columns 
was  carried  out  with  9-in. 
by  IJ-in.  boarding,  and 
clamped  with  4-in.  by  1-in. 
battens. 

FORMS  FOR  SILOS 


A  typical  silo  erected  at  the 
"Hovis"  Mill,  Vauxhall,  is 
shown  by  Figs.  290  and  291. 
It  is  difficult  properly  to  re- 
inforce a  rectangular  corner, 
and  for  this  reason  the 
columns  at  the  intersections 
of  the  silo  walls  are  on  the 
angle,  as  this  makes  the 
strongest  job. 

The  moulds  for  the  walls  of  each  bin  were 
formed  in  eight  sections,  each  with  planed 
1  J-in.  vertical  boards  3  ft.  6  in.  high,  secured 
to  top  and  bottom  horizontal  ledgers  2  in. 
by  7  in.,  and  a  2-in.  by  5-in.  middle  rail ; 
placed  against  these  were  planks  rebated 
to  receive  the  ends  of  the  4-in.  by  7-in. 


NOTE     FLOORING    LAID    DIAGONALLY 
OVER.   RIBS'.  OM  THI5  FLOOR.  WAS 
MID  THE  fOfins  FOR-  COFFERINQ 

FOR/1  roe 


Figs.  292  and  293. — Vertical  Section  and  Plan  of  Centering  for  Dome 
of  Wesleyan  Hall,  Westminster 


192 


. 


FORMS   AND    CENTERINGS 


193 


by  9  in.  timbers  bolted  together  with 
horizontal  cills  and  cross  braces,  similar  to 
a  gantry  for  a  derrick. 

The  art  of  disposing  timber  to  form  a 
centre  capable  of  supporting  the  weight  of 
the  material  placed  on  it,  and  ascertaining 


irizontal  stays  by  which  the  forms  were 
kept  in  position,  the  proper  distance  apart, 
and  which  in  turn  were  cross-strutted  with 
4-in.  by  6-in.  stays.  The  forms  at  the  angle 
column  were  kept  in  position  with  split 
2-in.  by  7-in.  vertical  and  2-in.  by  7-in. 
angle  struts  placed  horizontally,  two  in  the 
height  of  the  form.  The  forms  between 
two  adjoining  compartments  were  kept  the 
proper  distance  apart  at  the  head  by  long 
bolts  as  shown.  The  concreting  was  carried 
up  in  heights  of  2  ft.  at  a  time. 

The  forms  for  the  hopper  at  the  bottom 
of  the  silo  consisted  of  IJ-in.  dressed  boards 
on  2-in.  by  5-in.  raking  struts  carried  down 
to  the  platform  and  cross  braced  with 
battens.  The  upper  inside  boards  were 
laid  as  the  concreting  proceeded. 

Forms  for  circular  silos  are  constructed 
as  described  later  for  circular  tanks. 

Ffg.  294. — Centering  for  Dome  at  Annapolis 
New  Academy 

the  scantling  required,  demands  greater 
knowledge  than  the  ordinary  builder  usually 
possesses  ;  but  fortunately,  in  Great  Britain, 
at  any  rate,  centres  usually  err  on  the  side 
of  safety. 

The  following  examples  will  give  a  fair 
idea  of  the  nature  of  dome  centering  com- 
monly met  with  in  practice. 
Dome  of  Westminster  Hall. — At  the 
Wesleyan    Hall,    West- 
minster, is  probably  the 
finest  example  of  a  re- 
inforced  concrete  dome 
to    be   found   in   Great 
Britain,  its  span  being 
109    ft.   over  the    pen- 
dentives.     The   shell  of 
the  dome  is  only  4|  in. 
thick,    strengthened    by 
16  vertical    ribs  and    7 

I\^ ^/  ||  || jjj  horizontal  ribs  or  rings, 

Fig  296  Fig.  297  three    of     which    latter 

Figs.  295  to  297. -Centering  for  Octagonal  Dome  Circular  are    reinforced    to    take 

in  Section  the    thrust.       For    this 

the  bottom  main  rib  is 
ORMS  FOR  DOMES  reinforced  with   an   area  of  32  in.  of  steel. 

The  centering  requisite  for  erecting  domes  These  cross  ribs  form  an  ornamental  coffer- 
is  formed  of  numerous  uprights  carried  up  ing  which  is  very  effective.  The  centering 
to  the  springing  of  the  arch  from  a  firm  requisite  for  building  this  dome  was 
footing  below,  and  consists  either  of  scaffold  carried  on  a  platform  which  was  built 
poles  spaced  at  8  ft.  to  10  ft.  centres,  and  up  from  the  concrete  floor  of  the  hall, 
braced  with  ledgers  and  cross  braces.  In  Planks  were  laid  on  the  floor,  and  at  from 
other  cases,  the  uprights  are  built  of  3  in.  8  ft.  to  10  ft.  centres  uprights  consisting  o 


JOU2. 


13 


194 


REINFORCED    CONCRETE 


three  3-in.  by  9-in.  timbers  bolted  together 
were  erected,  together  with  horizontal 
ledgers  and  cross  braces.  On  the  top  of 
these  standards  were  laid  ledgers  which 
supported  the  working  platform  of  heavy 
planks.  Between  the  central  uprights  in 
the  middle  of  the  hall,  the  lift  was  erected, 
this  carrying  material  right  from  the  bottom 
to  the  top  of  the  building,  openings  being 
left  in  the  floor  for  this  purpose.  The  plat- 
form was  at  the  level  of  the  main  cornice  at 
the  springing  of  the  dome.  The  ribs,  which 


eight-rib  rolled  steel  joist  framework.  The 
filling  in  was  on  the  Columbian  system  of 
reinforced  concrete  by  cruciform  bars  sus- 
pended to  the  ribs  by  stirrups.  An  internal 
and  external  shuttering  was  used.  The 
inner  shuttering  was  nailed  to  the  horizontal 
radiating  ribs  placed  1  ft.  6  in.  to  2  ft.  apart, 
resting  on  triangular  brackets  fixed  to 
3-in.  by  7-in.  uprights  and  raking  struts,  the 
bottom  of  the  latter  being  secured  to  planks 
laid  on  the  floor.  A  portion  of  the  external 
shuttering  sufficient  for  a  part  of  the  dome 


Figs.  298  and  299.— Reinforcing  the  Large  and  Small  Half  Cupolas  of  the  Poti  Cathedral 


consist  of  two  l£-in.  by  9-in.  pieces,  bolted 
together,  were  entirely  floored  over  diagon- 
ally, this  flooring  supporting  the  forms  for 
the  coffering  slabs  and  beams,  treated  in 
every  respect  similarly  to  floors.  These  ribs 
were  supported  by  struts  and  uprights  con- 
sisting of  two  3-in.  by  9-in.  timbers  bolted 
together.  Lagging  was  only  erected  to 
support  the  underside  of  the  slab.  The 
upper  face  of  the  slab  concrete  was  worked 
up  from  the  springing  by  trowel  work.  The 
centering  is  clearly  shown  in  Figs.  292  and 
293. 

Dome   of  "Morning  Post"   Building, 
London. — This    was    constructed    on    an 


was  made  on  the  ground,  it  being'formed  of 
horizontal  ribs  shaped  to  the  contour  of 
both  plan  and  section.  These  ribs  were 
braced  by  3-in.  by  7-in.  boards.  The  dome 
was  concreted  in  sections  between  the  steel 
ribs,  the  concrete  being  rammed  from  the 
top,  the  same  portion  of  shuttering  being 
used  for  this  purpose.  When  the  lower 
portions  were  concreted  the  shuttering  was 
raised  by  wire  guys. 

A  20-ft.  Saucer  Dome.  —  The  forms 
for  a  saucer  dome  20  ft.  in  diameter,  sup- 
ported on  two  sides  of  a  barrel  vault,  which 
was  carried  out  at  the  chapel  of  St.  Charles 
College,  Netting  Hill,  London,  are  very 


FORMS    AND    CENTERINGS 


.teresting.  A  platform  was  erected  at  the 
level  of  the  springing  of  the  arch.  From 
this  platform,  resting  on  the  planks,  were 
raised  :3-in.  by  7-in.  uprights  wedged  up  to 
support  at  the  base  of  the  saucer  dome  a 
circular  ring  or  plate  formed  of  three  thick- 
nesses of  1^-in.  boarding,  which  in  turn 
supported  the  bottom  ends  of  3-in.  by  9-in. 
radiating  ribs  shaped  to  the  contour  of  the 
dome.  On  these  ribs  were  nailed  close- 
boarded  lagging.  The  arches  to  the  barrel 

,ults  were  supported  on  spliced  short  ribs 


enough  for  it  to  harden,  then  raised  2  ft.  and 
clamped  in  the  new  position.  The  moulds 
for  the  outer  form  were  first  put  in  place,  and 
the  inner  forms  were  hung  from  them.  This 
was  carried  up  gradually  until  the  men  were 
practically  working  on  a  floor  before  the 
crown  was  reached.  The  base  of  the  dome, 
which  was  69  ft.  in  diameter,  overhung  the 
main  walls  below  about  7  ft.  all  round,  the 
diameter  of  the  main  building  being  83  ft. 
Thus  heavy  corbelling  was  necessitated  to 
carry  the  bottom  ring  of  the  dome. 


Figs.  300  and  301. — Centerings  for  Belfry  and  Arches  of  the  Poti  Cathedral 


measuring  3  in.  by  7  in.,  supported  at  the 
joints  by  2-in.  by  5-in.  struts,  the  ends  of 
which  rested  on  a  3-in.  by  7-in.  tie  wedged 
up  from  the  platform.  The  pendentives 
were  filled  in  with  3-in.  by  9-in.  shaped 
ribs. 

^  Dome  of  Annapolis  New  Academy. — 
The  dome  at  the  new  academy  at  Annapolis, 
U.S.A.,  shown  by  Fig.  294,  is  a  wonderful 
piece  of  construction.  Its  peculiarity  is 
that  no  permanent  centering  was  used,  the 
mould  and  the  supporting  frame  being  built 
upwards  as  the  work  progressed,  receiving  its 
support  from  the  work  already  done.  The 
forms  were  kept  on  the  concrete  only  long 


The  cathedral  at  Poti,  Kussia,  is  described 
in  a  later  chapter,  but  it  is  convenient  to 
give  here  a  number  of  photographs  showing 
the  construction  of  the  dome  (see  Figs.  298 
to  305). 

Octagonal  Dome.  — Figs.  295  to  297 
show  the  centering  used  for  an  octagonal 
dome,  which  is  circular  in  section,  at  a 
mortuary  chapel  in  New  York.  Fig.  296 
includes  a  part-plan  of  the  ribs.  The 
main  section  is  through  the  angle  ribs,  a 
section  through  the  common  ribs  being 
shown  adjoining.  The  main  dimensions 
of  the  centering  are  figured  in  the  draw- 
ings. 


FORMS    AND    CENTERINGS 


197 


FORMS  FOR  TALL  CHIMNEYS 

The  most  economical  material  for  chimney 
shafts  of  circular  section  is  reinforced  con- 
crete, and  the  simplest  forms  for  shaft  con- 
struction consist  of  two  sets  of  inner  and 
outer  moulds,  each  3  ft.  high  (see  Fig.  306), 
held  together  by  means  of  latches  which 
can  be  readily  undone  to  enable  the  forms  to 
be  taken  away  easily.  When  the  concrete  is 
filled  in  to  the  top  of  the  upper  ring  the 
bottom  set  of  moulds  is  released  and  placed 


Fig.  306. — Centering,   etc.,  for   Chimney  at 
Northfleet 

on  the  top  of  the  upper  moulds,  left  in 
position  and  safely  held  there  by  the  frictional 
resistance  of  the  concrete  ;  this  course  is 
repeated  section  by  section  till  the  whole  is 
complete.  A  circular  wooden  gauge,  made 
of  two  f-in.  layers,  is  placed  6  ft.  above 
the  level  of  the  top  form  to  hold  the  rods  in 
true  alignment,  and  it  is  raised  as  the  work 
proceeds. 

As  the  work  is  carried  up  from  the  inside, 
only  a  light  scaffold  is  required,  built  up 
section  by  section  every  4  ft.  or  5  ft.,  the 
framing  of  each  section  consisting  of  four 
uprights  to  support  a  square  platform  of 


stout  planks,  holed  in  the  centre  for  hoist- 
ing the  materials  by  bucket.  Double-ring 
chimneys  are  carried  up  at  the  rate  of  one 
form,  and  single-ring  chimneys  at  the  rate 
of  two  forms  a  day. 

The  Ransome  System.— The  moulds 
and  false-work  used  by  the  Eansome  Com- 
pany in  the  United  States  for  chimney 
shafts  are  much  more  complicated.  A 
square  scaffold  tower  is  raised  in  the  centre 
of  the  chimney  with  four  4-in.  by  6-in. 
corner  posts,  and  1-in.  by  6-in.  cross  braces 
with  2-in.  by  10-in.  horizontal  braces  every 
4  ft.  or  5  ft.,  and  built  up  a  few  feet  in 
advance  of  the  construction  ;  the  scaffold 
tower  supports  a  platform  of  horizontal 
planks.  From  the  tower  project  beams, 
and  from  these  the  inner  and  outer  moulds 
are  suspended  by  means  of  four  vertical 
rods  having  their  upper  ends  threaded  and 
engaging  in  screw  wheel  bearings  so  that  the 
lower  moulds  can  be  raised  or  lowered. 
On  the  top  of  the  form  mould  on  the  outside, 
light  brackets  strutted  out  also  support  a 
working  platform.  To  avoid  removing  the 
cross  beams  at  the  head  for  each  set  of 
moulds,  telescoping  scaffolding  is  built  in- 
side. Inside  the  chimney  a  staging  is  sus- 
pended from  the  tower  from  which  the 
workmen  place  and  tamp  the  concrete. 
The  shell  moulds  are  12  ft.  high.  The  six- 
sided  lower  platform  is  for  the  workmen 
who  finish  the  outer  surface  of  the  concrete. 

FORMS    AND    CENTERING  FOR    ARCHES 
AND  BRIDGES 

In  designing  and  building  forms  and 
centering  for  arches,  care  must  be  taken  that 
the  centering  is  framed  up  strongly  enough 
to  take  the  weight  to  come  upon  it ;  that, 
while  the  material  is  used  economically,  the 
work  must  be  capable  of  being  easily  taken 
down  ;  that  the  footing  is  strong  enough 
to  support  the  verticals  without  settlement 
when  fully  weighted  ;  that  the  lagging  round 
the  centering  is  strong  enough  to  take  the 
weight  of  the  arch  ring  without  deflection ; 
that  the  centering  is  properly  held  up  in 
position  by  folding  wedges,  these  allowing 
the  centering  to  be  lowered  very  gradually 
so  that  the  safety  of  the  executed  work 
can  be  tested  before  finally  striking. 

In  general,  the  forms  for  arches  for  rein- 
forced concrete  are  substantially  the  same 
as  for  masonry  and  brickwork,  and  present 
no  difficulty  to  anyone  acquainted  with  the 
latter.  The  lagging  round  the  centering 


198 


REINFORCED    CONCRETE 


should  be  dressed  to  a  uniform  thickness, 
and  to  a  smooth  surface. 

Centerings  for  Flat  Bridges.— Concrete 
bridges  may  be  classified  under  two  heads, 
namely,  flat  bridges  and  arch  bridges.  Flat 
bridges  are  either  straight  flat  slabs  or  com- 


2  *4 


The  false-work  is  well  shown  in  the  photo- 
graph (Fig.  310). 
Centerings   for  Arch  Ring  Bridge.— 

The  centering  for  a  40-ft.  span  arch  ring 
bridge  is  shown  by  Figs.  311  and  312. 
8-in.  by  8-in.  piles  are  driven  in,  sawn 


ELEVATION. 


Fig.  307. — Centering  for  Flat  Bridge 


bined  slabs  and  girders,  and  are  adaptable 
for  spans  up  to  40  ft.  The  centering  for  a 
bridge  of  this  latter  type  is  shown  by  Fig. 
307,  and,  as  it  bears  a  strong  resemblance 
to  beam  and  floor  forms  already  described, 
it  needs  no  further  explanation. 

Centerings  for  Arched  Bridges.— 
Arched  bridges  are  constructed  with  or  with- 
out ribs.  If  the  arches  are  reinforced  with 
steel  joists  or  lattice  girders,  they  are  known 
as  Melan  Arches,  and  the  centering  is  partly 
hung  from  the  reinforcement,  as  shown  in 
Fig.  308. 

Arched  Roof  Principal  at  Hammer- 
smith Baths. — Fig.  309  shows  the  forms  for 
an  arched  principal  in  reinforced  concrete  at 
Hammersmith  Baths,  London,  one  of  the 
first  of  the  kind  to  be  carried  out  in  Great 
Britain.  In  accordance  with  the  usual 
practice,  a  working  platform  was  carried 
up  to  the  springing.  The  ribs  supporting 
the  forms  were  out  of  2-in.  by  7-in.  stuff, 
in  short  lengths  supported  at  the  junctions 
by  raking  and  vertical  struts  braced  hori- 
zontally with  2-in.  by  7-in.  ledgers.  On 
the  ribs  were  laid  2-in.  by  4-in.  lagging,  on 
which  was  nailed  1-in.  boarding  bent  to  the 
contour  and  forming  the  bottom.  The  sides 
were  formed  of  IJ-in.  boarding  clamped 
with  1-in.  by  3-in.  battens,  and  were  sup- 
ported on  the  2-in.  by  4-in.  projecting  lag- 
ging. The  sides  were  battened  also  on  top 
to  keep  the  forms  the  proper  distance  apart. 


off  near  the  top,  transverse  runners  are 
bolted  to  the  piles,  and  large  folding  wedges 
are  placed  between  the  tops  and  the  trans- 
verse 10-in.  by  10-in.  beams,  which  support 
the  weight  of  the  centering  above.  The 
centering  consists  of  a  set  of  caps  or  trans- 
verse beams  resting  on  the  wedges  above  the 


Fig.  308.— Centering  for  Melan  Arched  Bridge 

pile  caps,  some  shaped  bearers  notched  on 
and  supported  by  the  upper  transonic  beams, 
and  finally  of  a  closely  laid  lagging  resting 
on  the  bearers.  The  last-named  are  of 
varying  size,  depending  upon  the  distance 
between  the  verticals  and  the  weight  to  be 
carried.  For  arches  having  spans  up  to 


Fig.  309. — Elevation  of  Centering  for  Arched  Principal,  Hammersmith  Baths 


Fig.  310.— View  of  Centering  for  Arched  Principal,   Hammersmith  Baths 


199 


200 


REINFORCED    CONCRETE 


100  ft.  these  bearers  are  from  2  in.  to  4  in. 
wide  and  12  in.  to  14  in.  deep,  and  spaced 
from  1  ft.  6  in.  to  3  ft.  centre  to  centre. 
The  upper  surface  of  the  bearers  must  be 
curved  to  fit  the  curvature  of  the  under 


the  work,  when  the  centering  was  removed, 
was  found  to  be  only  T3F  in. 

The  main  arch,  of  about  259  ft.  span 
with  a  rise  of  about  87  ft.,  is  designed  for 
fixed  ends  without  hinges.  The  roadway  is 


Figs.  311  and  312.—  Centering  for  Arch  Ring  Bridge 


surface  of  the  arch,  and  the  bearers  must  be 
braced  laterally  by  1-in.  by  6-in.  bridging. 
The  lagging  consists  of  f-in.  tongued  and 
grooved  pine  or  2-in.  spruce  with  bevelled 
edges.  When  the  bearers  are  far  apart,  the 
lagging  may  have  to  be  as  thick  as  4  in. 

Teufen   Bridge.— The  bridge  at  Teufen, 
Switzerland  (see  the  photographs,  Figs.  315 


22  ft.  7  in.  wide,  and  is  more  than  216  ft. 
above  the  river.  The  arch  voussoir  is 
3  ft.  11  in.  at  the  crown  and  6  ft.  11  in.  at 
the  abutments.  To  carry  the  centering  for 
this  arch,  twelve  verticals  six  rows  deep  were 
driven  below  the  river  bed  and  carried  up 
above  the  springing  as  a  platform.  Each 
vertical  or  standard  was  formed  of  two 


i  :i 


1 


DETAIL  AT  B. 


Fig.  313. — Part  Elevation  and  Detail  of  Centering  for  Bridge  at  Teufen 


and  507)  designed  by  such  an  undoubted 
authority  as  Prof.  Morsch,  presents  points 
of  great  interest.  The  centering,  Fig.  313, 
designed  by  the  professor,  is  a  model  to  be 
safely  followed  in  similar  situations,  as  the 
permanent  deflection  at  the  completion  of 


12-in.  by  12-in.  balks  bolted  together  and 
stayed  with  ledgers  and  cross  braces,  both 
longitudinally  and  transversely,  at  every 
29  ft.  of  height.  On  top  of  these  standards 
were  placed  9-in.  by  12-in.  ties  cleated 
with  angle  irons  on  the  under-side  to  the 


FORMS    AND    CENTERINGS 


2OI 


verticals ;  a  12-in.  by  4-in.  by  40-in.  sole-piece 
was  bolted  to  the  ties,  on  which  was  laid 
two  sand  boxes,  except  on  the  end  row 
adjoining  the  abutments.  A  similar  sole- 
plate  rests  on  the  sand  boxes,  and  supports 
the  tie  beam  of  the  arch  centering  proper. 
Pieces  of  channel  iron  were  employed  to 


from  4  ft.  to  5  ft.  wide  and  the  whole  width 
of  the  bridge  were  left  unconcreted ;   these 


Fig.  314. 


-Detail    of  Bridge  Centering 
(see  A,  Fig.  313) 


distribute  the  pressure  of  the  vertical  and 

horizontal  timbers  on  to  the  sole-plates  and 

cross  beams  of  the  scaffolding  direct  on  to 

the  verticals,  and  special  steel  stirrup  ties 

held  in  the  feet  of  the  struts,  as  shown  in 

!Fig.  314.     The  main  struts  were  10-in.  by 

10-in.,  and  the  secondary  and  small  struts     sections  of  the  ring  were  strutted  apart  by 

9  in.  by  9  in.  and  5  in.  by  7  in.  respectively,      heavy  balk  timbers,  as  shown.    After  the 


Fig.  315. — View  of  Part  of  False-work  for 
Bridge  at  Teufen 


VERTICALS 


8*8  + 

6*4 


Figs.  316  and  317.  —  Centering  for  Almandares  Bridge,   Havana 

The  struts  supported  the  shaped  stringers     ring  had  taken  its  proper  bearings,  the  con- 


and  laggings,  each  stringer  being  composed 
of  two  13J-in.  by  3j-in.  planks.  The  arch 
ring  was  concreted  in  sections,  and  portions 


creting  of  the  omitted  parts  of  the  arch  ring 
was  completed. 
Arch  bridges  formed  of  a  series  of  arched 


Fig.  318.— Centering  for  Meadow  Street  Bridge,  Pittsburg 


STEEL  ROLLERS 


'•Ll^,  ANCHOR/ BOLT3 


Figs.  319  and  320.— Centering  for  Bridge  of  233-ft.  span 


203 


u 


204 


FORMS    AND    CENTERINGS 


205 


ribs  supporting  slabs  have  the  ribs  generally 
first  erected  on  such  centering  as  has  been 
described  for  the  Teufen  Bridge.  For  com- 
pleting the  slabs  after  the  ribs  have  hardened 


BRACES  2*8 


Fig.  327.— Centering  for  Bridge  of  80-ft.   span 

somewhat,  false-work    similar   to    that    de- 
scribed for  floors  is  used. 

Seventeen  examples  of  bridge  centering 
culled  from  the  best  Continental  and 
American  practice  are  shown  in  outline  or 
in  photographic  view  by  Figs.  316  to  332, 
the  sizes  of  the  main  timbers  being  given  as 
far  as  obtainable. 

SUSPENDED  CENTERING  FOR  BRIDGES 

In  the  building  of  bridges  over  running 
streams  great  difficulties  have  been  experi- 
enced owing  to  the  difficulty  of  erecting  the 
centering  and  owing  to  the  serious  settle- 
ments of  the  centering  that  frequently  take 
place.  American  engineers  found  that  the 
running  water  washed  round  about  the  base 


as  at  Teufen,  and  the  voussoirs  between 
these  sections  not  yet  concreted  in.  The 
voussoirs  were  raised  to  their  true  position 
by  screw-jacks,  which  had  to  be  continually 
adjusted  to  counteract  the  sinking 
still  going  on.  This  led  to  the  intro- 
duction of  flexible  suspended  center- 
ing, and  the  particular  design  about 
to  be  described  was  used  in  Chick- 
ahominy  Eiver  Bridge,  Eichmond, 
Va.,  U.S.A.  Fig.  333  shows  cables 
hung  over  the  piers  and  anchored  at 
the  abutment  of  the  arches.  At- 
tached to  these  cables  are  the 


Fig.  328.— Centering  for  Bridge  of  110-ft. 
span 

hangers  which  support  the  centering  shown 
in  Figs.  334  and   335.     In   order  to   pass 


)0>JZ 


Figs.  329  and  330.— Centering  for  Bridge  at   Deer  Park,   U.S.A. 


of  the  wood  piles  and  destroyed  their  vertical 
friction,  in  one  case  causing  a  settlement  of 
12  in.  Disaster  would  have  occurred  had 
not  the  arch  been  constructed  in  sections, 


the  cables  through  the  piers  or  abut- 
ments to  anchorage,  a  small  pipe  or  box 
with  proper  slope  is  embedded  in  the  abut- 
ments when  constructing  them. 


206 


REINFORCED    CONCRETE 


The  arch  can  be  raised  or  lowered  to  the 
proper  curvature  by  lengthening  or  shorten- 
ing the  hangers,  by  means  of  turnbuckles,  or 
by  differential  pulleys  for  very  heavy  loads. 


Fig.  331.— Centering  for  Flat  Bridge  of 
42-ft.  7-in.  span 

It  is  usual  to  set  up  the  arch  ribs  in  sections. 
The  key  spaces  are  then  filled  with  concrete, 
embedding  the  reinforcement  and  binding 
the  arch  sections  together.  The  arch  ribs 
being  completed,  the  cables  and  centering 
may  be  taken  down  and  any  superstructure 
desired  built  on  the  top  of  the  rib. 

Figs.  334  and  335  illustrate  a  panel  of 
suspended  centering  which  is  constructed 
of  wood  firred  on  top  with  shaped  member. 
The  furring  member  can  be  changed  to  suit 
n  arch  of  different  radius.  The  longitudinal 


adopted  in  Ireland  for  a  foot  bridge  172  ft. 
long,  and  150  ft.  above  the  level  of  the  sea. 
The  ribs  were  made  in  four  portions  on  the 
shore,  and  supported  by  overhead  cables  in 
position  as  before  described  till  the  key 
spaces  were  filled  in.  The  floor  was  shuttered 
in  situ,  being  supported  on  the  completed 
ribs. 

Figs.  336  to  338  are  photographs  which 
further  illustrate  the  method  of  building 
a  suspended  flexible  centering.  Near  the 
abutments  are  frames  or  towers,  from  the 
top  of  which  hang  cables  anchored  in  any 


SPAN  88'  6" 

Fig.  332.— Centering  for  Flat  Bridge  of 
88-ft.  6-in.  span 

suitable  way.  Wire  rope  hangers,  depending 
from  the  cables,  carry  cross  members  of  wood, 
iron  or  steel,  upon  which  rest  longitudinal 


Fig.  334. — Elevation  of  Panel  of  Suspended  Centering 


CABLE 


-CABLE 


CABLE 


i.*"-'*^'  •••••'  '•j.'./i  '• 
-^CONCRETE  RIB  : 

v  .•^-"s',-'  ••  \  •.  .  •.- 

J 

f 

fifASMPe  -tF£ET 

1 

^ 

I 

1 

Fig.  333.— One  of  the  Three  Spans  of  the 
Chickahominy  River  Bridge 


Fig.  335.— Section  of  Panel  of 
Suspended  Centering 


members   are   2-in.  by  12-in.  planks,  over- 
lapped  at   the   crossbars   and   notched   to 
receive  them.    The  cross  bars  are  3-in.  gas 
barrel,  4  ft.  long. 
A  similar  style  of  construction  has  been 


wood  or  steel  shapes  so  attached  that  the 
whole  centering  is  flexible,  and  any  curve 
may  be  imparted  to  it  merely  by  varying  the 
lengths  of  the  hangers  ;  for  the  finer  adjust- 
ments, the  hangers  have  turnbuckles.  The 


Fig.  336. — Gables  supporting  Concrete  Sections,  showing  Key  Spaces  to  be  filled  in 


Fig.  337.— Arch  Ribs  before  Striking  Suspended  Centering 


207 


208 


REINFORCED    CONCRETE 


end  sections  preferably  rest  on  the  abutments, 
the  bearings  being  hinged.  The  previously 
moulded  voussoir  sections  of  the  concrete 
arch  ribs,  or,  instead,  the  moulds  themselves, 


smaller  ones  are  constructed  by  joining  short 
lengths  of  concrete  pipes  together  and 
sealing  the  joints,  so  making  one  continuous 
pipe.  It  is  not  good  practice  to  mould  pipes 


Fig.  338. — Arch  Ribs  Formed  in  Suspended  Centering 


may  next  be  laid  in  place,  being  supported 
by  the  side  pieces.  The  approximate  lengths 
of  the  hangers  is  determined  in  advance,  the 
final  adjustments  being  made  by  means  of 
the  turnbuckles  or,  in  the  case  of  very 
heavy  work,  by  means  of  differential  pulleys. 
Targets  hang  from  the  cross  bars  on  supports 
of  different  lengths,  and  when  they  all  are 
at  the  same  level,  it  is  known  that  the 
flexible  centering  has  been  brought  to  its 
proper  form.  An  article  by  Philip  Aylett, 
in  Concrete  (Vol.  VII.,  pp.  24-35), 
should  be  consulted  for  further  particulars, 


of  less  than  3  ft.  diameter  in  situ  owing  to 
the  difficulty  of  devising  suitable"^  forms. 
When  more  than  3  ft.  in  diameter,  the  pipes 
can  be  easily  moulded  into  shapes  which 
have  more  stability  and  efficiency  than 
have  those  of  circular  cross  section.  By 
giving  concrete  pipes  a  broad,  flat  base, 


\ 
RlW 

%'BOLTS 

Fig.  339.— Centering  for  6-ft.  Sewer 

it  s  author  being  the  designer  of  the  examples 
of  flexible  centering  here  illustrated. 

FORMS  FOR  PIPES  AND  SEWERS 

The   larger  concrete   sewers   moulded  in 
place  are  practically  monolithic,  while  the 


SHEET 
IRON 


Fig.   340.— Centering  for  8-ft.   6-in.   Sewer 

they  have  a  better    bearing  on  the   foun- 
dation. 

Fig.  339  shows  the  centering  for  a  sewer 
6  ft.  in  diameter,  constructed  in  8  ft.  lengths, 
16  ft.  being  done  at  one  operation.  To 
facilitate  striking  the  forms  they  are 


FORMS    AND    CENTERINGS 


209 


constructed  in  four  portions.  The  ribs  are 
2  in.  thick,  secured  to  each  other  by  f-in. 
bolts  at  32-in.  centres,  and  braced  by  2-in. 
by  6-in.  bracing  secured  to  the  f-in.  bolts. 
The  top  section  is  secured  in  position  by 


shown.  The  12-in.  by  1^-in.  ribs  at  18-in. 
centres  are  secured  by  2-in.  by  8-in.  braces. 
Fig.  342  shows  the  form  for  an  arched 
culvert  5  ft.  wide.  After  the  soil  has  been 
prepared  for  the  concrete,  the  4-in.  by  4-in. 


Fig.    341.— Centering    for 
Conduit  at   Jersey  City 


Fig.   342. — Centering  for  5-ft.   Arched  Culvert 


cills  should  be  set  and  braced  with 
1-in.  by  6-in.  braces.  The  inner 
arch  form  is  then  wedged  up  from 
the  cills.  The  ribs  are  shaped  out 
of  1^-in.  stuff  and  placed  3  ft.  apart 
centre  to  centre,  this  necessitating 
thick  lagging  (2  in.  by  3  in.) ;  1-in. 
stuff  would  do  if  the  ribs  were  only 
2  ft.  apart.  On  the  outer  side,  1-in. 
by  4-in.  or  1-in.  by  6-in.  horizontal 


rn 


Fig.  343.— Centering   for 
8-ft.  Arched  Culvert 

12-in.  by  6-in.  wedges. 
There  is  f  in.  clearance 
between  the  top  section 
and  the  side  section  for 
dropping  the  centre.  2-in. 
by  2-in.  laggings  are  fixed 
to  the  ribs  •  and  covered 
with  No.  27  gauge  sheet 
iron. 

Fig.  340  shows  the  form  for  a  sewer 
8  ft.  6  in.  wide.  This  pipe  was  constructed 
in  six  sections,  and  made  in  7-ft.  lengths. 

Fig.  341  shows  the  centering  for  the 
conduit  for  water  supply  for  Jersey  City, 
which  is  constructed  in  seven  sections  as 

14 


Fig.  344.— Form    for 
Small  Box  Culvert 


boards  are  laid  to  2-in.  by  4-in.  braces  and 
uprights  set  to  the  splay  of  the  extrados, 
as  shown. 

Fig.  343  shows  the  form  for  an  8-ft.  arched 
culvert.  The  1-in.  boards  of  the  wall  form 
are  secured  by  2-in.  by  4-in.  verticals  staked 


2IO 


REINFORCED    CONCRETE 


into  the  ground  'and  strutted  against  dump- 
ing left  in ;  a  '4-in.  by  4-in.  cill  is  fixed, 
and  on  top  of  this  is  laid  the  wedges,  at 
least  3  in.  deep,  to  facilitate  the  removal 
of  the  arch  forms,  which  are  framed  of  1  £-in. 


Fig.  345.— Collapsible  Steel   Centering  for  Sewer 

stuff,  2  ft.  3  in.  apart.  The  side  forms  can 
then  be  easily  removed  after  the  arch  forms 
are  struck. 

Fig.  344  shows  the  form  for  a  small  box 
culvert  24  in.  wide,  the  top  splay  giving  it 
an  arch  effect  cheaply.  The  outer  form  of 


the  wall  consists  of  2  in.  by  10  in.  boards 
secured  to  4  in.  by  4  in.  posts  driven  into 
the  ground  and  braced  by  1  in.  by  3  in.  cross 
pieces.  The  inner  forms  are  of  1  in.  boards 
propped  up  by  2  in.  by  4  in.  head  pieces, 
and  cills  and  posts,  which  are  placed  without 
nailing.  The  form  is  struck  by  pushing  the 
2  in.  by  4  in.  head  pieces  off  the  side  posts. 
Collapsible  Steel  Centering. — A 'great 
deal  of  sewer  work  is  carried  out  by  the 
Blaw  collapsible  steel  centering,  which  con- 
sists of  flexible  steel  plates  bent  cold  to  the 
proper  radius  and  stiffened  with  channel 
ribs  (as  in  Fig.  345).  The  sections  are  5  ft. 
long,  and  held  together  at  the  ends  by 
staples  passing  through  slots  on  the  male 
end  of  each  section.  The  centres  are  held 
in  position  by  two  tension  rods  attached 
by  a  turnbuckle,  these  being  also  used  for 
collapsing  the  centres.  The  rivets  being 
countersunk  present  a  fair  face  to  the 
concrete.  The  following  briefly  describes 
the  method  of  their  use.  A  dish  is  prepared 
in  the  trench,  on  which  to  place  the  centre. 
The  concrete  is  then  placed  in  position  and 


Fig.  346. — Conduit  at  Woolwich,   showing  Collapsible  Steel  Centering  in  Use 


FORMS    AND  CENTERINGS                                    211 

jammed  ;    after  it  has  set,  the  centres  are  FORMS  FOR  TANKS 

ollapsed  by  means  of  the  turnbuckles,  which  Form    for    Square    Cistern. — Figs.  347 

ase  them  from   the  concrete  without  any  and  348  show  forms  for  the  erection   of  a 
arring    or    hammering    whatever.      When 


4*2- 


p  

F  —  n 

\-r-t-  . 

4*1 

\ 

r^M 

)  ' 

9 

J           [ 

4*2 


Figs.   347  and  348.— Form  for  Square  Tank 


be  centres  are  collapsed  they  are  dropped 
rollers  and  pulled  along  ready  for  the 
ext  section.  When  the  centres  are  very 
irge  they  are  mounted  on  small  wheels  run 
n  light  rails.  The  centering  is  adapted  to 


small  cistern  4  ft.  6  in.  by  4  ft.  6  in.  by  6  ft. 
high.  1-in.  dressed  boards  are  cut  to  the 
exact  inside  dimensions,  namely  4  ft.  6  in.,  on 
two  sides,  and  4  ft.  4  in.  on  the  other  two 
sides,  and  they  are  held  in  position  by 


Figs.  349  to  352.— Form  for  Circular  Tank 


Afferent   shapes  for  all   kinds   of  conduits 
ad  for  concrete  pipe  moulds. 
Fig.    346   is   a   reproduction  of  a  photo- 
j:aph  clearly  showing  the  Blaw  co  lapsible 
i  :ntering  in  use. 


,6*6 


J  ROD 


2  *4    LAGGING 

Fig.  351 

2-in.   by   4-in.   posts  strutted  across 
with   1-in.   horizontal   boards   at  top 
and    bottom.      Eight    2-in.    by    4-in. 
posts  at  the  corners  are  set  on  the 
ground,     and     held     in 
position  by  2-in.  by  4-in. 
inclined  strutting  nailed 
to  planks  on  the  ground 
secured  by  stakes.      The 
inner    and    outer   forms 
are  kept  the  proper  dis- 
tance    apart    by     1-in. 
battens    nailed  to    both 
posts.       The  posts  may 
be  much  higher  than  the 
tank    in    preference    to 
cutting      the     material. 
The  outside  boards  can 
be  allowed  to  project  beyond  the  corners 
and  thus  save  needless  cutting. 

Form  for  Circular  Tank.— Figs.  349 
to  352  show  forms  for  a  circular  water  tank 
24  ft.  in  diameter.  Six  segments  of  60 


212 


REINFORCED    CONCRETE 


degrees  each  form  the  circumference,  and 
they  are  4  ft.  high.  There  are  two  bands 
of  spliced  2-in.  by  8-in.  planks  at  top  and 
bottom  shaped  to  the  circle,  and  nailed  to 
these  at  8-in.  centres  are  2-in.  by  4-in. 
laggings  to  which  are  secured  No.  22  gauge 
galvanised  iron.  The  yoke,  formed  of 
6-in.  by  6-in.  verticals,  2-in.  by  6-in.  braces 
and  adjusted  by  a  turnbuckle,  is  the  means 
by  which  the  outer  and  inner  forms  are 
kept  the  proper  distance  apart,  it  also 
enabling  the  forms  to  be  raised  as  the  work 
proceeds. 

Form  for  Gasholder  Tank.— Fig.  353 
represents  a  section  through  the  walls  of 
the  gasholder  tanks  at  Dubuque,  erected  for 
the  Key  City  Gas  Company.  The  bottom  is 
about  5  ft.  below  the  outside  ground  level, 
and  the  walls  rise  to  a  height  of  21  ft.,  being 
18  in.  thick  at  the  base,  tapering  to  12  in. 
at  the  top.  The  concrete  was  first  laid  over 
the  entire  bottom  16  in.  thick,  but  increased 
under  the  walls  to  a  thickness  of  2  ft.  6  in. 


round  the  heads  of  the  posts  was  also  dished 
out  in  order  that  the  posts  could  be  cut 
down  in  the  striking  of  the  scaffolding,  and 
the  holes  filled  up.  The  whole  264  ft. 
circumference  of  the  tank  wall  was  filled 
up  to  the  level  of  the  top  of  the  form  in  one 
operation.  Forms  were  used  to  make  a 
mortise  and  tenon  joint  between  one  day's 
work  and  the  following  day's.  Between  the 
outer  side  of  the  wall  and  the  earth,  up  to 
the  higher  ground  level,  concrete  was  filled 
in,  well  rammed  and  tamped,  and  it  was 
relied  upon  as  a  support  to  the  base  of  the 
wall.  Two  2-in.  by  12-in.  planks  nailed  on 
top  of  temporary  posts  were  laid  radiating 
to  the  tank  wall.  On  top  of  this  two  layers 
of  1-in.  by  6-in.,  spliced  in  short  lengths  and 
shaped  to  the  radius  of  the  wall,  were  fixed, 
braced  in  position  with  2-in.  by  6-in.  up- 
rights and  2-in.  by  4-in.  inclined  braces. 
The  scaffolding  supporting  the  wall  forms 
on  the  inside  of  tank  consisted  of  4-in.  by 
6-in.  cills  laid  on  the  ends  of  the  post,  which 
projected  8  in.  above  the  floor  ;  from  these 
were  erected  double  2-in.  by  4-in.  verticals, 
with  4-in.  by  6-in.  ledgers  and  2-in.  by  6-in. 
cross  bracing.  The  outside  alternate  pilasters 
and  piers  were  built  up  with  two  sides 
complete,  and  the  third  side  added  as  the 
work  proceeded.  The  shuttering  to  the 
outside  wall  was  similar  to  that  on  the 
inside  and  was  fastened  in  position  be- 
tween and  under  the  edges  of  the  pilaster 
forms  and  lag  screwed  to  the  4-in.  posts. 
The  edges  of  the  inside  shutters  were  bolted 


4x4,      6*1   PLANKS 


.2*8 


~sr 
1 


\r- 

3A  HOLE 
'/2  POO  WITH  3" EYE  END 

Pit.  ^54 


2*12 


Fi«.  353 
Figs.  353  and  354.— Form  and  Centering  for  Gasholder  Tank 


As  the  foundation  was  uncertain,  piles  were 
driven  4  ft.  6  in.  apart  to  support  the  floor. 
The  last  named  was  roughly  dished  at  the 
bottom  of  the  wall  in  order  to  give  a  good 
key  both  to  the  bottom  of  the  wall  and 
to  support  the  scaffolding  of  the  wall  forms, 
which  were  fixed  to  temporary  posts  driven 
where  shown  to  carry  them.  The  concrete 


to  the  sides  of  the  inside  2-in.  by  4-in. 
verticals  of  the  scaffolding,  and  the  little 
triangular  spaces  between  the  shutters  were 
filled  up  with  2-in.  vertical  strips  of  steel 
on  a  backing  strip  of  wood  1  in.  square. 

The  forms  were  raised  3  ft.  at  one  opera- 
tion, by  blocks  attached  overhead  to  a  cap 
piece,  and  as  the  forms  were  4  ft.  high 


FORMS    AND    CENTERINGS 


213 


there  was  an  overlap  of  1  ft.  on  the  finished 
concrete.     The  forms  were  cleaned  and  oiled 


after  every  operation.    Fig.  354  shows  the 
inside  shutter  for  the  tank. 


Fig.  355. — Form  for  Gasholder  Tank  at  San  Sebastian 


Fig.  356. — Gasholder  Tank  at  San  Sebastian 


A  photographic  view  of 
the  forms,  etc.,  for  a  gas- 
holder tank  at  San  Sebas- 
tian is  presented  by  Fig. 
355,  the  finished  tank 
being  shown  by  Fig.  356. 

Form  for  Rectangu- 
lar Reservoir. — In  an 
example  of  a  rectangular 
reservoir  having  walls 
strengthened  by  pilasters, 
the  slab  roof  was  carried 
on  rib  beams.  In  excav- 
ating for  the  walls,  the 
footings  were  made  slightly 
larger  and  a  2-in.  plank  on 
edge  inserted.  After  plac- 
ing concrete  for  the  foot- 
ings, horizontal  shutter- 
ing of  2-in.  board,  dressed 
inside  and  propped  up  by 
4-in  by  4-in.  uprights 
placed  at  4-ft.  centres,  was 
erected.  For  the  curved 
corners  thin  boards  were 
bent  to  the  required  curve 
and  secured  to  the  up- 
rights, the  latter  being  2 
ft.  apart.  The  posts  on 


214 


REINFORCED    CONCRETE 


the  outside  were  propped  up  by  raking 
struts  and  wires  passing  through  the  wall 
to  the  inside,  there  being  braces  connected 
to  the  adjoining  columns  and  walls,  and  also 
struts  extending  to  a  proper  bearing.  As 
before,  the  two  sides  of  the  pilasters  were 


are  cheaper,  as  they  require  no  upkeep. 
Figs.  357  and  358  show  the  form  for  square 
posts.  A  plank  is  laid  as  a  base  dressed 
on  the  upper  side,  and  on  this  are  set  two 
tapering  pieces,  2  in.  by  6  in.  at  one  end 
tapering  to  2  in.  by  4  in.  at  the  other. 


5-0 


I 

jgl 

pJA 

Fig.  357.— Elevation,  Plan,  and  End  View  of 
Form  for  Tapered  Square  Posts 


m 


Fig.  358. — Cross  Section 
(enlarged)  through  Form 
for  Tapered  Square  Posts 


6-lS1 


1 


Q 


Fig.  359. — Plan  and  Elevation  of  Multiple 
Form  for  Tapered  Square  Posts 

completely  formed,  and  the  third  side  added 
as  the  work  proceeded.  The  slab  ceiling 
and  beam  forms  were  carried  out  as  previ- 
ously described  for  floors,  with  2-in.  boards 
dressed  on  one  side,  both  to  the  beams 
and  slab.  The  beam  forms  and  slab  cen- 
tering were  erected  on  4-in.  by  4-in.  up- 
rights resting  on  the  finished  floor  beneath 
and  cross  braced  at  intervals. 

FORMS  FOR  FENCE  POSTS 

Reinforced  concrete  fence  posts  have  been 
used  with  success,  and  although  their  first 
cost  is  more  than  wood,  in  the  end  they 


Fig.  360. — Section  of  Form  for 
Triangular  Posts 


These  tapering  pieces  are  held  in  position 
without  nails  by  the  side  fillets  which  are 
nailed  to  the  base  board.  The  tops  of  the 
sides  are  stayed  as  shown.  Fig.  359  shows 
a  multiple  form  for  moulding  four  fence 
posts  at  a  time,  the  posts  being  tapered 
four  sides.  The  divisions  separating  the 
posts  are  slipped  in  between  cleats  at  each 
end,  and  held  in  place  by  wedges  against 
blocks  nailed  to  the  platform.  The  illus- 
tration is  self-explanatory.  Fig.  360  shows 
a  mould  for  triangular  posts.  The  frame  is 
hinged  so  that  posts  having  different  angles 
can  be  made  in  it. 


Systems   Described 


IN  this  chapter  will  be  found  descriptions 
of  a  number  of  the  best-known  modern 
systems  of  reinforced  concrete,  based  on 
information  supplied  by  various  firms  in 
the  industry.  It  will  thus  be  understood 
that  where  special  claims  are  made  for  this 
or  that  arrangement,  it  is  simply  the  in- 
ventor's or  owner's  point  of  view  that  is 
presented.  The  explanations  have  been 
kept  as  concise  as  possible,  since,  as  most 
readers  already  know,  the  firms  responsible 
for  the  various  systems  issue  lengthy  and 
well  -  illustrated  descriptions,  which  may 
generally  be  had  for  the  asking,  and  it  is 
therefore  a  simple  matter  for  the  reader  to 
follow  up  any  special  claims  or  other  points 
of  interest  here  briefly  stated  which  may 
invite  his  further  study. 

ARMOURED   TUBULAR    FLOORING 

A-  typical  floor  on  this  system  consists 
essentially  of  three  parts  (see  Figs.  361  and 
362),  the  concrete  webs  A,  coke-breeze  tubes 
B,  and  a  concrete  top  layer  c.  The  webs  are 
reinforced  with  a  ribbed  corrugated  bar  as 
shown,  and  these,  together  with  coke-breeze 
tubes,  which  are  9  in.  long,  are  delivered 
complete  on  the  site.  First  the  webs  are 
placed  in  the  proper  position,  then  the  tubes 
are  put  in  between,  and  the  top  layer  of 


transverse  wires,  the  two  being  electrically 
welded  together  at  the  points  of  inter- 
section. The  wire  fabric  can  be  supplied  as 
bright  steel  or  galvanised  ;  either  of  the 
strands  may  be  of  any  thickness,  from 
No.  4  to  No.  12  gauge  (Imperial  Standard 
Wire  Gauge),  but  No.  5  is  the  heaviest  that 
can  be  used  when  both  strands  are  of  the 
same  size.  The  longitudinal  wires  are  spaced 
3  in.  apart,  and  the  standard  spacing  of  the 
transverse  wires  is  12  in.,  16  in.,  or  18  in. 
The  maximum  number  of  longitudinal  wires 
is  24.  The  standard  maximum  width  is  72  in. 
The  greatest  width  possible  is  97  in.,  and 
the  wire  fabric  is  supplied  in  rolls  or  sheets, 
a  roll  of  the  heavy  gauge  containing  150  ft., 
and  of  the  lighter  gauges  200  ft. 

COIGNET  SYSTEM 

Beams  and  Slabs. — The  essentials  of  a 
Coignet  beam  are  shown  by  Fig.  363,  in 
which  A  indicates  the  principal  bars  subject 
to  tension,  B  secondary  bars  working  in  com- 
pression, C  stirrups  to  resist  shear,  D  principal 
bars  in  floor  slab,  and  E  secondary  bars  in 
floor  slab.  The  principle  of  the  beam  is  the 
introduction  into  the  compressed  portion  of 
the  concrete  of  the  secondary  bars  over  which 


Figs.  361   and  362. — Armoured  Tubular  Floor 


concrete  is  spread  over  the  whole.  The 
ceiling  side  of  the  floor  may  be  left  as  laid, 
or  may  be  finished  by  plastering. 

CLINTON    SYSTEM 

Slabs. — This  system  depends  on  the  use 
of  an  electrically  cross-welded  steel  wire 
fabric,  consisting  of  a  series  of  parallel 
longitudinal  wires  spaced  at  certain  dis- 
tances apart,  and  held  at  intervals  by 


the  stirrups  or  shear  members  are  hooked. 
The  stirrups  or  shear  members,  which  con- 
nect together  the  concrete  working  in  com- 
pression and  the  bars  working  in  extension, 
increase  the  resistance  to  shearing  and  also 
the  compressive  resistance  of  the  concrete. 
Where  the  stirrups  pass  round  the  bars,  they 
are  fastened  by  means  of  annealed  wire  so 
as  not  to  be  disturbed  when  the  concrete 
is  applied.  The  skeleton  reinforcement  is 


2l6 


REINFORCED    CONCRETE 


prepared  in  advance,  and  stacked  till  ready 
for  use.  As  the  spacing  of  the  stirrups  is 
proportional  to  the  shearing  stresses,  they 
are  closer  together  near  the  supports  where 
the  maximum  shearing  stresses  occur.  Much 
importance  is  attached  to  the  fact  that  both 
stirrups  and  the  main  bars  are  of  round 
section,  so  that  the  line  of  contact  between 
them  is  a  mere  line. 

Fig.  363  illustrates  an  early  type  of  beam, 
which  has  been  largely  superseded  by  that 
shown  by  Fig.  364. 

In  a  later  type  of  Coignet  beam  (Fig.  364) 


Fig.   363.— Coignet 
Beam  and  Slab 


Fig.  366. — Section 
through  Coignet 
Column 


Fig.  367  and  368.— 
Base  of  Coignet 
Column 


the  lower  bars  shown  in  the  previous  figure 
are  replaced  by  a  group  of  bars  of  smaller 
diameter,  their  ends  being  bent  upwards  at 
an  angle  of  45  degrees  and  hooked  over  the 
top  bar.  The  sectional  area  of  the  reinforce- 
ment is  greatest  in  the  middle  portion  of  the 
beam  where  the  bending  moments  are  higher, 
and  the  bars  are  spaced  closer  together  near 
the  supports  as  in  the  previous  example. 
The  bars  are  bound  together  with  annealed 
wire  as  before.  The  bars  do  not  lie  in  con- 
tact one  with  another,  there  being  a  space 
of  at  least  J  in.  between  them  into  which 
the  concrete  flows. 


The  reinforcing  of  the  floor  slab  has  already 
been  shown  in  Fig.  363.  The  principal  bars 
D  are  of  such  diameter  and  spacing  as  will 
best  resist  the  tension,  the  secondary  bars 
E  merely  distributing  the  efforts  more  easily 
on  the  principal  ones.  At  alternate  inter- 
sections of  the  bars,  they  are  bound  together 
with  annealed  wire.  Coignet  floor  slabs 
usually  vary  in  thickness  between  3  in.  and 
6  in.  ;  but  should  the  thickness  exceed  this, 
a  double  reinforcement  is  advised,  stirrups 
connecting  the  upper  and  lower  bars  at 
intervals. 


Fig.   365. — Beam  Supporting  Floor  Slab 
Centering 


Fig.  364. — Coignet  Beam  Reinforcement 
Consisting  of  Group  of  Small  Bars 


Fig.  369  and  370.— Coignet  Pipe  or 
Conduit 


Coignet  beams  may  be  prepared  in  advance 
and  transported  and  placed  in  position  like 
steel  girders.  They  are  made  in  a  horizontal 
wooden  mould,  the  principal  bars  being  left 
protruding  for  about  a  foot  at  each  end  so 
that  they  may  be  fixed  on  to  their  supports 
by  means  of  cement  grout.  Fig.  365  shows 
how  the  ready-made  beams  may  be  used  to 
support  the  centering  for  the  floor  slabs.  The 
upper  ends  of  the  stirrups  project  so  as  to 
form  a  mechanical  bond.  This  method  is 
seldom  used. 

Columns. — A  simple  type  is  shown  by 
Fig.  366.  In  this,  the  vertical  bars  are 


SYSTEMS    DESCRIBED 


217 


bound  by  horizontal  or  spiral  hoops  or  ties 
to  keep  them  in  their  proper  position  and 
also  to  resist  a  bursting  tendency.  The 
hoops  are  fastened  to  the  verticals  by  means 
of  annealed  wire.  The  footing  of  the  pillar 
where  it  is  necessary  to  spread  the  weight 
takes  the  form  shown  in  Figs.  367  and  368. 

Walls. — As  these  are  to  resist  lateral 
pressure  only,  and  not  intended  to  support 
any  vertical  load,  this  being  carried  by 
beams,  a  meshwork  of  vertical  and  horizontal 
bars  is  placed  in  the  centre  of  the  wall  to 
facilitate  concreting,  the  intersections  being 
bound  together  with  wire. 

Pipes. — The  reinforcement  takes  the 
form  shown  in  Figs.  369  and  370,  and  the 
pipe  or  sewer  is  constructed  in  lengths  of 
about  4  ft.  or  5  ft.,  and  connected  to  the 
next  section  by  means  of  joint  and  spigot 
as  shown  ;  alternatively,  the  pipes  may  be 
concreted  in  continuous  length. 

Piles. — Practically,  these  are  pillars 
made  in  a  horizontal  mould  instead  of  a 
vertical  one.  Generally,  as  in  Fig.  170 
(p.  133),  they  are  circular  in  section,  with 
two  flat  longitudinal  surfaces  which  assist  the 
proper  guiding  of  the  pile  during  driving. 

CONSIDERE  SYSTEM 

Beams  and  Slabs. — This  system  is  well- 
nigh  devoid  of  fanciful  methods  of  shaping 
and  arranging  the  reinforcement,  and  is 
claimed  to  rest  entirely  on  a  scientific  basis. 
In  the  case  of  minor  beams,  the  tensional 
reinforcement  takes  the  form  of  round  bars, 
some  of  which  are  horizontal  throughout, 
and  others  are  bent  up  at  the  ends.  Where 
a  compression  member  is  required,  a  spiral 


supports  at  each  end,  there  being,  in  addition, 
round  bars  which  cross  the  other  reinforce- 
ments at  right  angles.  In  large  girders 
and  trusses,  as  employed  in  bridge  con- 
struction, the  tension  members  are  rein- 
forced in  the  manner  already  described, 
whilst  the  compression  members  are  rein- 
forced with  longitudinal  bars  and  spiral  coils. 

Columns. — Columns  are  generally  cir- 
cular, octagonal,  or  square  in  cross  section, 
and  the  vertical  reinforcements  are  straight 
round  bars  from  4  in.  to  6  in.  apart,  whilst 
the  transverse  reinforcements  consist  of 
round  rods  in  the  form  of  coils  wound 
spirally  round  the  vertical  reinforcements, 
the  ends  of  the  coils  being  bent  inwards. 

Piles.  —  Considere  has  devoted  much 
attention  to  the  construction  of  reinforced 
concrete  piles  on  scientific  principles.  His 
general  form  of  pile  is  octagonal,  with 
longitudinal  reinforcing  rods,  generally  num- 
bering about  eight,  inside  a  continuous  spiral 
winding  of  round  steel  rods,  the  pitch  of  the 
spiral  being  about  2  in.  at  the  middle  and 
diminishing  to  1£  in.  at  both  head  and  foot. 
The  head  is  of  cylindrical  shape  and  for 
about  4  in.  is  bound  with  steel  coils  closely 
pitched,  this  reinforcing  the  head  to  such 
an  extent  that  it  will  not  fracture  under 
the  driving  shock,  even  though  a  cap  or 
dolly  is  not  used.  Illustrations  showing  the 
application  of  the  Considere  system  in  prac- 
tice will  be  found  on  pp.  358  to  360. 

CORR  BAR 

This  is  an  American  system  of  supplying 
beam  and  column  reinforcement  already 
made  up  and  suitable  for  immediately  placing 


Fig.  371.— Corr  Bar  Beam  "Unit" 


Fig.  372.— Types  of 
Corr  Bars 


coil  of  round  steel  is  inserted  near  the  top 
surface.  Occasionally  beams  are  addition- 
ally reinforced  against  shear  by  means  of 
thin  steel  rods,  which  are  lapped  round  the 
tension  and  compression  bars.  In  addition 
to  the  above,  Considere  sometimes  inserts 
an  extra  reinforcement  (a  spiral  coil  of 
round  steel  laid  nearly  horizontal)  in  that 
part  of  the  concrete  which  is  in  compression 
near  the  supports  of  a  continuous  beam. 
The  slab  reinforcements  between  the  beams 
consist  of  round  bars  bent  up  over  the 


in  the  forms.  Each  made-up  reinforcement  is 
called  a  "  unit,"  and  a  typical  construction 
is  shown  by  Fig.  371,  this  type  having  been 
designed  to  give,  by  means  of  continuous 
stirrup,  an  efficient  web  reinforcement.  The 
unit  for  a  round  column  consists  of  two  or 
more  verticals  with  a  continuous  hooping  of 
cold-drawn  wire.  The  American  company 
responsible  for  this  system  has  introduced 
a  variety  of  special  reinforcements,  including 
deformed  bars  (see  Fig.  372)  and  a  kind  of 
expanded  steel  known  as  "  Corr  Mesh." 


218 


REINFORCED    CONCRETE 


DENTILE   SYSTEM 

Floors. — By  means  of  this  system,  floors 
are  constructed  having  the  advantages  of 
terra-cotta  tile  floors  and  those  of  reinforced 


portions  fitting  together  to  form  a  closed 
hollow  block.  The  reinforcement  can  be 
placed  in  both  directions.  In  the  case  of 


Fig.  373.— Dentile  Floor  with  Mitre  Tiles  F»&  376- — Diamond  Mesh  Expanded  Metal 


concrete.  Briefly,  upon  the  forms  are  placed 
hollow  tile  blocks  closed  on  all  six  sides  and 
having  projecting  flanges,  which  space  them 
at  proper  intervals.  In  the  channels  so  left 
the  reinforcing  bars  are  placed,  the  concrete 
being  then  applied  and  a  floor  surface  being 
obtained  by  spreading  a  layer  of  concrete 


Fig.  374.— Dentile  Floor  with  Bridge  Tiles 

right  over  the  tiles.  Essentially,  then,  the 
floor  consists  of  hollow  tiles  with  ribs  of 
reinforced  concrete  between  them  united  at 
the  top  by  a  thin  slab  of  plain  concrete.  On 
the  ceiling  side  there  is  a  continuous  sur- 
face of  tile.  Clear  spans  up  to  32  ft.  have 
been  built  and  tested,  while  for  light  loads, 


Fig.  375.— Dentile  Floor  with   "L"  Tiles 

and  where  surrounding  conditions  are  favour- 
able, the  spans  may  be  as  much  as  40  ft. 
Three  shapes  of  tile  are  used  :  mitre  tiles 
(Fig.  373),  consisting  of  four  wedge-shaped 


the  bridge  tile,  shown  by  Fig.  374,  the 
reinforcement  is  in  one  direction  only.  With 
L  tiles,  (Fig.  375),  the  reinforcement  can  go 
both  ways.  Small  flat  filling  pieces  are  used 
to  put  in  the  square  corners  between  the 
tiles.  In  all  cases  the  under-sides  of  the  tiles 
are  roughened  so  as  to  take  the  ceiling 
plaster. 

EXPANDED    METAL   SYSTEM 

Expanded  metal  consists  of  rolled  steel 
of  various  thicknesses,  cut  and  expanded 
by  machinery  into  meshes  of  various  shapes, 
the  material  being  obtainable  in  a  number 
of  different  strengths  or  weights.  Diamond 
'  mesh  is  shown  by  Fig.  376.  There  are  thirty 
varieties  of  this  material,  the  variations  being 
in  the  size  of  mesh,  thiclcness  of  strands,  the 


Fig.  377.— Rib  Mesh  Expanded  Metal 

weight  per  yard  super,  etc.  The  rib  mesh 
is  shown  by  Fig.  377,  this  being  made  in 
five  varieties  ;  however,  for  concrete  work 
the  diamond  mesh  material  is  chiefly  used. 


SYSTEMS    DESCRIBED 


What  is  known  as  the  expanded  steel  bar 
is  a  bar  from  which  a  series  of  meshes  has 
been  expanded  (see  section,  Fig.  378).  It 
is  claimed  to  be  a  complete  unit,  comprising 
tension  and  shear  reinforcements,  and  as  its 
members  are  rigidly  connected,  they  cannot 


219 


e — by  a  number  of  stirrups  clipped 
firmly  around  the  tension  bars  as  shown  in 
Fig.  380  and  bent  over  at  the  top  so  as  to  be 
anchored  into  the  concrete,  and  they  also 
insert  other  bars  straight  for  the  greater 
part  of  their  length  and  bent  upwards  at 


Fig.   378. — Section  of  Expanded 
Steel  Bar 


"*~~  Expanded  Metal  Lathir.g 


esh  Expanded  Steel" 


get  out  of  place  during  the  con- 
creting. When  expanded  metal  is 
used  in  connection  with  these  _E 
bars,  the  ends  of  the  sheets  may 
be  wired  to  or  bent  over  and 
lapped  on  to  the  web  of  the  bars, 
thus  securing  continuity  of  rein- 
forcement. 

Floors. — When  these   bear  on 

the     walls,     it    is     necessary     to        Fi&  379.— Four  Types  of  "Expanded  Metal"  Floors 
leave    an    offsett,    corbel,    chase, 


***»  Expanded  Metal  Lathing 


Reinforced  Concrete  Bel 
Expanded  Sttel  Bars 


Ik  mesh  Expanded  Steel 


Reinforced  Concrete  Bea 
Reinforcing  Rod: 


or  over-sail.  The  method  of  constructing 
the  floor  with  expanded  metal  is  described 
on  page  152,  and  Fig.  379  presents  sec- 
tional drawings  of  a  few  typical  styles  of 
construction. 

Other  Applications. — Expanded  metal 
has  been  found  applicable  to  a  wide  range 
of  work  which  it  is  quite  unnecessary  to 
describe  in  detail. 

HENNEBIQUE  SYSTEM 

Beams. — Scientific  designing  rather  than 
the  use  of  steel  of  special  shape  or  high- 
carbon  content  is  the  essential  of  this  system. 
Thin  bars  or  rods  and  stirrups  of  steel  are 
inserted  in  just  those  places  where  the  stress 
diagram  of  the  structure  shows  that  the 
resistance  of  the  concrete  requires  to  be 
supplemented.  The  experts  responsible  for 
the  system  believe  that  if  it  were  practicable 
to  apply  the  teachings  of  theory  to  their  full 
extent,  the  steel  reinforcement  employed 
would  be  in  the  form  of  fine  wires  receiving 
and  transmitting  stress  throughout  the  con- 
crete. They  therefore  endeavour  to  recon- 
cile theory  and  practice  by  using  moderately 
small  bars  as  the  main  reinforcement,  and 
small  rods,  wire,  or  steel  strips  as  the  con- 
necting or  auxiliary  reinforcement,  and  all 
these  are  in  forms  procurable  in  the  open 
market.  They  believe  that  a  beam  rein- 
forced simply  by  straight  tension  bars  near 
the  lower  face  would  not  be  satisfactory  in 
practice,  and  therefore  they  supplement 
those  bars — in  a  beam  of  the  most  simple 


each  end,  part  of  such  a  bar  being  shown 
by  Fig.  381.  Diagramatically,  a  simple 
beam  of  this  type  is  shown  in  Fig.  382, 
which  includes  a  longitudinal  section  and 
three  cross  sections.  The  bars  near  the 
lower  surface  adequately  reinforce  the 
middle  portion  of  the  beam,  and  the  inclined 
bars  and  the  vertical  stirrups  supply  the 
additional  reinforcement  necessary  at  the 
ends  of  the  beam  where  tension  occurs  at 
diagonal  planes.  The  company  emphasises 
their  belief  that  the  bars  and  stirrups  rein- 
force the  concrete  against  diagonal  tension 
rather  than  against  vertical  shear,  and  say 
that  what,  in  beam  tests,  is  commonly 
described  as  a  "  shearing  "  failure,  is  almost 
invariably  a  diagonal  tension  failure.  The 
bent-up  ends  of  the  bars  lie  across  the  lines 
of  rupture  near  the  supports  of  the  beam, 
and  afford  in  themselves  very  secure  anchor- 
age. The  spacing  of  the  vertical  stirrups  is 
such  as  to  provide  for  variations  of  stress  from 
point  to  point.  Being  vertical,  they  facili- 
tate the  operation  of  ramming  the  concrete 
without  risk  of  displacement,  and  being 
made  with  a  spring  clip  at  the  lower  end, 
they  are  automatically  held  in  position  on 
the  main  bars  without  either  wedges  or  ties. 
The  stirrups  form  an  effective  web  connection 
between  the  tension  and  compression  por- 
tions of  the  beam.  The  company  does  not 
believe  that  any  stress  or  vibration  developed 
under  working  conditions  can  possibly  over- 
come the  natural  adhesion  between  the  con- 
crete and  the  plain  round  bars,  but  the  ends 


22O 


REINFORCED    CONCRETE 


of  the  bars  are  flattened  and  opened  out  to 
form  secure  anchorage,  and  at  least  half  the 
bars  are  bent  up  towards  the  supports, 
further  to  ensure  perfect  security.  In  beams 
that  are  continuous  over  intermittent  sup- 
ports, the  ends  of  the  bent-up  bars  are 
carried  horizontally  across  the  supports  and 
terminate  near  the  points  of  contraflexure 
(see  Figs.  383  and  334).  The  inclined  bars 


dimensions.     The  double  stirrups  will   be 
noticed. 

Floors. — Hennebique  floors  are  simple 
combinations  of  beams  and  slabs.  The  main 
beams  receive  the  load  which  is  trans- 
mitted from  the  secondary  beams,  the  latter 
receiving  the  load  transmitted  from  the 
continuous  slab  connecting  all  the  beams  and 
receiving  the  super  load  placed  upon  the 


Fig.  381.- 


Fig.  380.  —Henne- 
bique Stirrup 
round  Tension  Bar 


-Hennebique  Tension  Bars 
and  Stirrups 


I         T 

B  D 


iUr 


3*cCion  on  A-O.  Section  on  CD  Section  on  C  r 

Fig.  382. — Simple  Hennebique  Beam 


385. — Beam    Reinforcement    with     Com- 
pression Bar  and  Double  Stirrups 


Fig.  387  and  388.— 
Base  of  Henne- 
bique Column 


Fig.  389.— 
Sheet  Piles 


z-  ai 


V 


Fig.  390.— 

Hollow 

Diaphragm 

Pile 


Figs.    383  and   384. — Hennebique    Beams    Continuous  over 
Intermittent  Supports 


Fig.  386. — Hennebique 
Column 


in  each  span,  and  the  horizontal  ends  of 
other  inclined  bars  projecting  from  the 
adjoining  spans,  then  resist  the  tensile 
stress  occurring  in  the  upper  part  of  con- 
tinuous beams  between  the  supports  and 
the  points  of  contraflexure. 

Beams  constructed  as  above  described  are 
the  most  economical  types,  but  it  is  some- 
times advantageous  to  employ  compression 
bars  near  the  upper  surface  as  shown  in 
Fig.  385,  which  type  of  beam  has  advant- 
ages when  a  very  heavy  load  has  to  be 
carried  by  a  beam  of  comparatively  small 


floor.  In  a  typical  instance  the  main  beams 
have  parallel  reinforcing  bars,  and  the 
secondary  beams  have  single  reinforcement. 
In  the  usual  type  of  Hennebique  floor 
half  the  slab  panel  on  each  side  of  each 
secondary  and  main  beam  constitutes  a 
compression  flange,  and,  with  the  beam, 
forms  a  beam  of  T-section.  The  longitudinal 
bars  and  stirrups  of  the  main  beams  pass 
into  the  superimposed  slab,  and  where  there 
is  compression  reinforcement,  this  is  entirely 
within  the  slab.  The  Hennebique  hollow- 
tube  flooring  is  mentioned  on  p.  332. 


SYSTEMS    DESCRIBED 


221 


Columns. — A  typical  Hennebique  column 
is  shown  by  Fig.  386,  the  vertical  reinforce- 
ment consisting  of  plain  round  bars,  and  the 
auxiliary  reinforcement  being  formed  of 
links  of  TVm-  steel  wire  applied  in  sets  of 
four,  one  link  passing  round  the  two  verticals 
on  one  side.  The  links  brace  the  longitu- 
dinal bars  and  support  the  concrete  later- 
ally. This  method  was  introduced  many 
years  ago,  and  has  given  satisfaction  in 
thousands  of  cases. 

Figs.  387  and  388  show  a  column  base,  the 
horizontal  steel  reinforcement  distributing 
the  force  transmitted  by  the  vertical  bars. 
The  lower  portion  of  the  concrete  is  rein- 
forced by  a  double  system  of  bars  laid  at 
right  angles  with  one  another  so  as  to  pro- 
vide for  the  tensile  stresses  caused  by  the 
bending  moment  developed  by  the  central 
load  and  the  vertical  reaction  of  the  ground. 
Kesistance  to  compression  is  provided 
entirely  by  the  upper  portion  of  the  con- 
crete, and  diagonal  tension  is  taken  by  the 
vertical  stirrups  applied  as  in  beams.  A 
column  base  of  this  description  virtually 
represents  cantilever  construction. 

Piles. — These  can  be  built  of  any  approved 
shape,  that  shown  on  p.  133  being  typical. 
The  longitudinal  bars  are  bent  inwards  at  the 
toe,  but  do  not  bear  upon  the  steel  driving 
point,  this  being  anchored  into  the  concrete 
by  four  straps  bent  over  at  the  upper  end, 
as  shown  on  p.  133.  The  transverse  ties, 
resembling  those  in  the  column  already 
described,  are  spaced  more  closely  near  the 
top  and  bottom  of  the  pile  than  in  the 
middle. 

Sheet  piles  for  retaining  walls  and  coffer- 
dams have  a  head  of  reduced  section  (see 
Fig.  389),  and  on  two  sides  of  the  pile  there 
is  a  semicircular  groove,  the  hole  formed 
by  the  grooves  in  contiguous  piles  being 
filled  with  cement  grout  to  prevent  the  per- 
colation of  water.  Thus  a  row  of  sheet 
piles  forms  a  watertight  wall.  The  hollow 
diaphragm  pile  (see  Fig.  390)  is  constructed 
on  the  principle  that  the  resistance  to 
driving  is  largely,  if  not  entirely,  due  to 
friction  between  the  external  surface  of 
the  pile  and  the  earth,  and  that  a  given 
volume  of  reinforced  concrete  employed  in 
the  form  of  a  hollow  cylinder  is  more  effective 
than  if  applied  as  a  solid  cylinder.  The 
reinforcement  is  practically  the  same  as  that 
of  the  solid  pile,  but,  in  addition,  diaphragms 
hold  in  place  a  consecutive  series  of  tubes 
each  about  4  ft.  long,  the  object  of  the 


tubes  being  to  form  the  hollow  core  of  the 
finished  pile. 

INDENTED    BAR    SYSTEM 

This  system  depends  entirely  on  the  use 
of  a  deformed  bar,  either  square  or  round 
in  section  (see  Figs.  391  and  392).  The 
square  bar  varies  from  J  in.  to  2  in.  square, 
and  from  -24  Ib.  to  13-6  Ib.  per  foot  run  in 
weight,  whereas  the  round  bar  varies  from 
fin.  to  1|  in.  in  diameter,  the  weight 
per  foot  run  at  these  sizes  being  -38  Ib. 
and  6'06  Ib.  respectively.  Strictly  speak- 
ing, the  bar  itself  is  the  "  system,"  the 
company  responsible  for  it  having  no 
special  -method  of  arranging  the  rein- 
forcement, but  preparing  their  designs 
in  accordance  with  approved  practice,  and 
supplying  the  indented  bars  to  the  building 
contractor  to  be  used  in  accordance  with  the 
designs  prepared  by  the  Indented  Bar  Coin- 


Fig.  391. — Square  Section  Indented  Bar 


Fig.  392. — Round  Section  Indented  Bar 

pany  or  by  independent  engineers.  They 
contend  that  their  bar  obtains  a  more 
reliable  grip  on  the  concrete  than  does  any 
smooth  bar.  The  adhesion  between  smooth 
bars  and  concrete  depends,  they  state,  upon 
the  cement  particles  entering  into  the  micro- 
scopical irregularities  in  the  surface  of  the 
steel.  When  a  steel  bar  is  subject  to  a 
tension  of  12,000  Ib.,  or  say  5  tons,  per 
square  inch,  it  extends  by  -004  in.  per  inch  of 
length,  this  causing  corresponding  reduction 
in  the  area  of  cross  section,  which,  although 
extremely  small,  is  sufficient  to  affect  the 
adhesion.  They  instance  a  number  of  tests, 
the  results  of  which  go  to  show  that  indented 
square  bars  sustain  1-9  times  the  stress 
required  to  pull  out  smooth  squares,  and 
indented  round  bars  3'7  times  the  stress 
required  to  pull  out  smooth  round  bars. 
Further,  it  is  stated  that  owing  to  the  fact 
that  at  a  stress  of  3  tons  per  square  inch  the 
extension  in  steel  is  greater  than  that  to 
which  concrete  can  be  subjected  without 
cracking,  it  appears  to  be  undesirable  when 
smooth  bars  are  used  to  allow  the  tension 
in  the  steel  to  exceed  that  amount.  With 


222 


REINFORCED    CONCRETE 


indented  bars,  however,  the  continuous  and 
positive  bond  provided  makes  it  permissible 
for  the  stress  in  the  steel  to  be  limited  only 
by  the  strength  of  the  material  used,  this 
enabling  a  steel  of  high  tensile  strength  to 
be  employed.  The  draft  regulations  of  the 
London  County  Council  propose  that  there 
should  be  a  mechanical  bond  between  the 
steel  and  concrete  in  cases  where  steel  used 
has  a  greater  strength  than  72,000  Ib.  per 
square  inch. 

When  the  company's  ordinary  specifica- 
tion is  adopted,  the  bars  are  of  rolled  medium 
steel  with  an  elastic  limit  of  50,000  Ib.  per 
square  inch,  with  an  extension  of  not  less  than 
15  per  cent,  in  a  length  of  8  in. ;  the  breaking 
strength  is  90,000  Ib.,  roughly  40  tons  per 
square  inch.  By  the  use  of  such  strong 
material  there  is  claimed  to  be  a  consider- 
able saving  in  the  weight  of  steel  required 
for  a  given  strength,  the  factor  of  safety 
can  be  increased  without  extra  cost,  and  the 
omission  of  bending,  splitting,  cranking,  and 
other  devices  commonly  used  for  anchoring 
smooth  bars  is  a  source  of  economy. 

JOHNSON'S  LATTICE  SYSTEM 

This  depends  on  the  use  of  a  steel  wire 
mesh  or  lattice  (see  Fig.  393),  manufactured 
in  sheets  or  panels,  and  in  rolls  6  ft.  wide  to 
about  120  ft.,  or  the  rolls  can  be  made  longer 
than  120  ft.  and  any  width  other  than  6  ft., 
and  the  spacing  can  be  varied  to  suit 
individual  requirements.^  It  is  unnecessary 


Fig.   393. — Johnson's  Steel  Wire  Lattice 

to  give  full  details  of  the  various  methods 
in  which  this  reinforcement  is  applied.  It 
is  sufficient  to  say  that  it  is  applicable  to 
very  many  purposes,  from  foundation  rafts, 
to  floor  slabs,  walls,  etc.  Figs.  394  to  397 
show  four  different  methods  of  supporting 
mesh-reinforced  floors. 

KAHN    SYSTEM 

Beams. — The  experts  responsible  for  this 
system  emphasise  the  fact  that  a  properly 
constructed  reinforced  concrete  beam  is  in 


reality  a  trussed  beam,  and  they  state  that 
stirrups  can  only  transfer  stress  to  the  main 
tensional  member  when  the  two  are  defi- 
nitely and  rigidly  connected  together,  the 
horizontal  reinforcement  taking  not  only  the 
stress  caused  from  adhesion  of  the  concrete 
to  it,  but  also  the  summation  of  the  hori- 
zontal components  of  the  strain  in  each 
of  the  diagonals.  The  principles  of  truss 
action  occur  (or  grow)  out  of  this.  The 


I  r^v_  _T^ 

— 

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Figs.  394  to  397. — Four  Methods  of  Supporting 
Mesh-reinforced  Floors 

company  does  not  believe  that  the  concrete 
surrounding  the  bars  will  prevent  the  loose 
stirrups  from  slipping,  and  they  therefore 
attach  the  shear  members  rigidly  to  the 
horizontal  reinforcement.  In  the  Kahn 
system  a  bar  with  two  projecting  wings  or 
stirrups  is  used,  the  stirrups  being  bent  up 
at  about  45  degrees,  so  as  to  cross  the  planes 
of  rupture  at  nearly  right  angles  (see  Fig. 
398).  At  the  centre  span,  where  the  hori- 
zontal tension  is  greatest,  the  web  is  left 
intact  on  the  bar,  and  there  serves  as  an 
additional  reinforcement  (see  Fig.  399).  The 
stirrups  are  part  of  the  original  metal  of 
the  bars,  being  merely  sheared  and  bent, 
whereas  in  the  original  Kahn  bar,  holes  were 
punched  in  it  and  the  shear  members 
fastened  in  these  holes.  The  Kahn  trussed 
bar,  it  will  be  understood,  is  sent  to  the 
site  complete,  ready  to  be  incorporated  in 
the  concrete  as  a  single  unit. 

The  bar  undoubtedly  possesses  great 
advantages  in  lintel  construction.  The  prin- 
ciple is  stated  as  follows  :  A  flat  arch  acts 


exactly  upon  the  same  principle  as  a  seg- 
mental  arch,  and  therefore' -if  a  section  of 
the  wall  above  an  opening  can  be  converted 
into  such  a  flat  arch, 'and  a  tie  member  suit- 


Fig.  398.— View  of  Kahn  Bar 

-4»- 


Fig.  399. — Section  and  Elevation  of  Kahn 
Trussed  Bar 


ably  placed  so  as  to  receive  its  thrust,  then 
that  section  of  the  wall  has  become  in  reality 
an  arch.  This,  in  short,  is  the  purpose  of 
the  Kahn  lintel,  wherein  the  Kahn  trussed 
bar  is  used  as  a  tie  member,  the  diagonals 
of  the  bar  taking  up  the  thrusts  of  the  arch. 
The  lintels  can  be  built  for  spans  as  great 
as  30  ft.  or  40  ft.  Tigs.  400  and  401  show 
the  application  of  the  Kahn  bar  in  lintel 
construction. 

Floors. — The  floor  slabs,  reinforced  with 
top  and  bottom  horizontal  bars,  the  lower 


Fig.   402. — Keedon   Beam   Reinforcement 


Fig.  403. — Keedon  Column  Reinforcement 

of  which  is  a  bar  of  the  type  already  des- 
cribed, form,  with  a  trussed  beam,  a  beam 
of  T-section. 

Columns. — The  Kahn  bar  is  also  applic- 
able to  column  construction.  The  prongs 
projecting  from  the  bars  (the  bars  are  here 


223 

used  as  vertical  members)  reach  diagonally 
across  the  column  and  tie  in  the  main  bars 
at  intervals  of  6  in.  to  12  in.  Some  Kahn 
pillars  have  helical  hooping. 


Figs.   400  and  401.— Lintel  Reinforced  with 
Kahn  Bar 

"Hy-rib"  is  a  sheet-mesh  reinforcement 
and  lathing  introduced  by  the  Kahn 
Company. 

KEEDON  SYSTEM 

The  peculiarity  of  this  system  is  the  use 
of  stirrups,  hoopings,  etc.,  which  are  looped 
to  pass  over  the  main  bars,  and  are  rigidly 
held  to  those  bars  by  means  of  wedges  or 
keys.  Beam  reinforcement  on  this  principle 
is  shown  by  Fig.  402,  and  column  reinforce- 
ment by  Fig.  403.  The  rigid  shear  members 


Fig.  404. 


-Column   and   Beam   Reinforcements 
Keedon  System 


or  stirrups  resist  the  diagonal  tension.  Thus, 
the  stirrups  and  hoopings  form  in  effect  a 
series  of  rigid  projections,  but  the  bars,  of 
course,  retain  their  original  cross  section, 
and  are  not  deformed  in  any  way.  All  the 
stirrups,  hoopings,  and  keys  are  inter- 


224 


REINFORCED    CONCRETE 


changeable,  and  the  work  of  assembling  them 
is  quickly  done.  The  main  reinforcements 
are  ordinary  merchantable  steel  bars.  Fig. 
401  shows  the  Keedon  system  applied  to  a 
column  and  beams. 


Fig.  405. — Lock -woven   Mesh 

Fig.  406. — Lock -woven  Mesh  Floor 

LOCK-WOVEN  MESH  SYSTEM 

This  depends  on  the  use  of  a  special  re- 
inforcement, consisting  of  steel  wires  woven 
together  at  right  angles  and  secured  at  the 
intersections  by  means  of  machine-made 


Fig.  408.— Floor  Slab  Supported  by  Four 
Columns,  Mushroom  System 

knots,  which  are  of  three  kinds,  as  shown  in 
the  composite  diagram,  Fig.  405.  The 
lengthwise  wires,  known  as  carrying  wires, 
are  heavier  than  the  transverse  or  distri- 
buting wires,  since  they  take  the  tensional 
stress,  whereas  the  other  wires  do  little  more 


than  keep  the  fabric  rigid  in  the  course  of 
laying  and  distributing  any  accidental  loads 
and  temperature  stresses.  The  fabric  is 
sent  out  in  rolls  or  in  the  form  of  sheets 
cut  to  size  and  shape.  The  firm  responsible 


Lock  Woven  Mesh 


Fig.  407. — Fireproof  Construction 
with  Lock-woven  Mesh 

for  the  system  emphasises  the  advantage  of 
using  the  close-fibre  steel,  which  is  drawn 
out  under  considerable  pressure,  rather  than 
a  material  in  which  the  fibres  have  been 
severed  by  slotting.  The  method  of  placing 


Fig.   409. — Head  of  Column,   Mushroom 
System 

in  position  reinforcements  of  this  type  is 
explained  on  p.  152  in  the  chapter  "The 
Erection  of  a  Reinforced  Concrete  Building." 
Fig  406  shows  a  type  of  floor,  with  alterna- 
tive methods  of  constructing  the  beams.  The 
reinforcement  is  carried  over  the  steel  joists 


SYSTEMS    DESCRIBED 


225 


and  sags  in  the  centre  of  the  floor,  so  that 
it  comes  close  to  the  under-surface  of  the 
latter.  It  is  obvious  that  Lock-woven  Mesh 
is  applicable  to  a  great  variety  of  reinforced 
concrete  constructions.  Fig.  407  shows 
methods  of  fireproofing  floor  girders  and 
columns  with  reinforced  concrete  casings. 

MUSHROOM   SYSTEM 

This  is  an  American  system,  in  which  the 
bar  reinforcement  to  the  columns  are  splayed 
out  radially  top  and  bottom  so  as  to  be 
thoroughly  bonded  into  the  floors.  As 
shown  in  Fig.  408,  there  are  four  belts  of 
rods  crossing  the  slab  from  column  to 
column,  and  splayed  out  over  the  supple- 


the  peculiar  formation  of  the  rods  around 
the  column  head,  and  is  claimed  to  simplify 
centering.  It  concentrates  the  maximum 
amount  of  reinforcement  around  and  over 
the  support  where  the  shear  is  the  greatest, 
and  eliminates  beams  and  ribs,  giving  a  flat 
ceiling  which  allows  of  freer  illumination 
from  the  windows  and  more  convenient 
placing  of  shafting  in  the  case  of  factory 
equipment.  The  system  has  been  built 
and  tested  for  nearly  all  spans  of  from  14  ft. 
to  30  ft.,  and  larger  spans  can  readily  be 
made.  Fig.  409  shows  the  head  of  a  column, 
and  is  an  example  taken  from  actual  prac- 
tice. The  columns  are  spaced  20  ft.  6  in. 
centre  to  centre  each  way,  and  the  floors 


big.  414 


Fig.  411.— Paragon 
Column,  Beam, 
and  Floor  Slab 


Fig.   410.— Paragon 
Stirrups 


mentary  cantilever  reinforcement  at  the 
top  of  the  column  (this  description  is  based 
on  one  by  the  inventor,  C.  A.  P.  Turner,  in 
his  "  Concrete  Steel  Construction,"  Part  I.). 
It  will  be  noted  from  the  diagram  that  in 
certain  areas  the  rods  are  only  one  layer  or 
belt  in  thickness,  and  a  practical  test  of  the 
construction  up  to  the  yield  point  of  the 
steel  after  the  concrete  has  thoroughly  set 
shows  the  development  of  cracks,  due  to  the 
stretch  of  the  steel,  approximately  along  the 
dotted  lines  shown  in  the  diagram,  these 
lines  therefore  showing  the  planes  of  greatest 
weakness,  and  between  them,  in  the  centre 
of  the  slab,  there  being  an  approximately  cir- 
cular flat  plate.  The  inventor  demonstrates 
that  these  lines  are  nearly  or  approximately 
the  points  of  maximum  moment  in  the  slab. 
The  Mushroom  system  is  so  called  from 

15 


Fig.  412.— Paragon     Figs.  413  and  414.— 
Column  Hoopings        Paragon        Helical 
Column  Wrappings 

are  designed  to  carry  150  Ib.  per  square  foot 
live  loads.  Each  column  is  reinforced  with 
eight  1-in.  round  steel  bars,  which  project 
4  ft.  at  the  top,  where,  however,  they  are 
bent  at  right  angles  over  a  f-in.  by  2-in. 
band  placed  just  above  the  bottom  of  the 
floor  slab.  The  rods  flare  radially  into  the 
slab  from  this  band,  and  extend  outwards 
to  a  distance  of  3  ft.  9  in.,  while  two  circles 
of  steel  rods,  8  ft.  4  in.  and  4  ft.  6  in.  in 
diameter  respectively,  rest  on  the  radial 
bars  and  are  wired  to  them.  The  reinforcing 
bars  in  the  floors  extend  well  over  each 
column  head,  and  run  parallel  and  diagonal 
to  the  lines  of  the  columns. 

PARAGON   SYSTEM 

Special  forms  of  stirrups,  hoopings,  and 
wrappings    are    the    peculiarities    of    this 


226 


REINFORCED    CONCRETE 


system.  Dealing  first  with  the  stirrup  for 
use  in  beams,  heavy  floors,  and  similar 
structures  subject  to  flexure  or  bending 
loads,  the  stirrups  are  relied  upon  to  resist 
the  web  or  shearing  stresses ;  they  lock 
tightly  to  the  main  bars,  and  are  claimed 
to  give  a  perfect  mechanical  bond  dis- 
tributing the  stresses  along  the  full  length 
of  the  beam  or  slab  to  ensure  that  the  con- 
crete and  steel  act  together  in  taking  the 
load.  They  can  be  made  to  lie  at  any  angle 
to  make  the  most  suitable  form  of  truss  and 
of  any  required  length.  They  are  so  shaped 


of  hoopings  are  shown  in  Fig.  412,  and  of 
these  the  first  and  last  may  be  referred  to. 
The  first  is  for  solid  work,  and  has  four  bars 
which  pass  through  the  loops  shown.  The 
arms  A  extend  across  the  area  enclosed  by 
the  hoop  and  diverge,  their  ends  being 
turned  and  bent  away  from  each  other.  The 
last  has  the  arms  curved  to  form  a  central 
ring  for  use  where  a  central  column  or  pipe 
is  to  be  enclosed. 

The  helical  wrappings  (Figs.  413  and  414) 
are  claimed  to  overcome  objections  to  which 
a  continuous  wrapping  is  liable — namely, 


Fig.  415.— Piketty  Beam 


Figs.  418  and  419. — Cross  Sections  of  Piketty 
Beam   with   Four  Rows  of  Bars 


Figs.    416  and  417. — Piketty  Beams  with  Two  and  Three 
Tension   Bars 


Figs.   420  and  421. — Square  Piketty   Column          Figs.  422  and  423.— Round    Piketty    Column 


as  to  give  a  minimum  metal-to-metal  con- 
tact, and  they  are  available  in  a  variety  of 
shapes,  in  which  Fig.  410  shows  the  prin- 
cipal five.  Fig.  411  is  a  comprehensive 
diagram  which  includes  a  beam  in  which 
the  stirrups  are  employed. 

Hoopings  are  used  in  columns,  piles,  etc., 
and  are  claimed  to  reinforce  the  core  in  the 
planes  between  the  hoops  against  the  bulging 
action,  for  which  purpose  they  have  inturned 
arms,  or  their  arms  are  twisted  together  to 
form  spiral  binds  which  lie  through  the 
centre  of  the  core.  Each  hoop  is  placed  so 
that  its  arms  lie  in  a  different  direction  from 
those  of  the  adjoining  hoops.  Five  styles 


that  the  concreting  cannot  be  seen  until  it 
reaches  the  top  of  the  column  form ;  that 
concrete  has  to  be  dropped  in  from  the  top, 
giving  the  larger  particles  a  chance  of  leaving 
the  finer  ones,  and  so  forming  a  porous 
space ;  and,  thirdly,  the  wire  wrapping  may 
be  injured  in  the  course  of  tamping,  and  so 
cause  the  whole  length  to  be  materially 
weakened.  The  Paragon  helical  wrapping 
has  been  sectionised,  the  ends  of  the  sections 
being  made  to  reinforce  the  core  in  the  form 
of  bonds  similar  to  the  hoopings  already 
described.  A  wrapping  is  threaded  by  means 
of  its  loop  over  the  end  of  a  bar,  fixed  in 
position,  and  the  concrete  placed  around  it, 


SYSTEMS    DESCRIBED 


227 


the  tamping  being  done  from  the  open  side 
of  the  form  and  consequently  providing  no 
excuse  for  injuring  the  wrapping.  The 
figure  illustrates  the  reinforcement  for  a 
circular  column,  but  square  columns  also 
can  be  made  by  this  method. 

PIKETTY   SYSTEM 

Beams  and  Slabs. — Paul  P.  Piketty, 
the  inventor  of  this  system,  considers  that 
double  reinforcement  is  absolutely  indis- 
pensable, for  reasons  which  are  pursued  at 
length  in  the  "  Handbook  "  describing  the 
system,  among  them  being  fastening  of 
stirrups — increase  of  compressive  resistance, 
and  resistance  to  secondary  tensile  stresses 
being  the  result  of  permanent  deformations 
in  concrete.  Fig.  415  shows  two  views  of 
the  reinforcement  of  an  ordinary  beam. 
There  are  two  series  of  bars,  namely, 
straight  bars,  and  bars  bent  up  at  a  third 
of  their  length.  To  resist  the  shearing 
stresses,  there  are  stirrups  connecting  the 
lower  with  the  upper  reinforcing  bar  and 
inclined  as  shown ;  their  ends  are  bent  over 
as  shown  in  the  cross  section,  and  all  bars  are 
fish-tailed  to  resist  any  tendency  to  longi- 
tudinal sliding.  The  beam  shown  by  Fig. 
416  is  on  the  same  lines,  but  the  lower  bar 
is  hooked  and  fish-tailed.  There  are  three 
series  of  bars  in  the  beam  shown  by  Fig.  417, 
whilst  Fig.  418  is  the  cross  section  of  a  beam 
with  four  rows  of  bars.  Fig.  419  is  the  cross 
section  of  a  beam  with  symmetrical  reinforce- 
ment. The  transverse  bars  are  in  tension, 
whilst  the  horizontal  links  hold  the  four 
rows  of  bars  together  and  increase  the  com- 
pressive resistance  of  the  beam.  Slabs  on 
this  system  have  both  lower  and  upper 
reinforcement,  the  bars  of  one  being  generally 
at  right  angles  to  the  bars  of  the  other. 

Columns. — Figs.  420  to  423  show  pillars 
or  columns  reinforced  on  the  Piketty  system. 
The  square  pillar  has  four  vertical  reinforce- 
ments, and  the  special  shape  links  will  be 
noted.  The  round  column  has  six  rein- 
forcements, the  links  being  on  the  same 
principle  as  in  the  square  column. 

OTHER  SYSTEMS 

Bonna  Pipes  and  Conduits. — These  are 
reinforced  with  ribbed  bars  formed  into 
longitudinals  and  spiral  coils,  the  two  being 
notched  together  and  tied  at  the  joints 
(see  also  p.  369). 

Chain  Concrete  Floors. — By  means  of 
special  clips,  round  steel  bars  laid  parallel 


in  the  floor  slabs  are  connected  together  to 
form  a  continuous  sheet. 

Columbian  Floors. — The  reinforcement 
comprises  ribbed  steel  bars  in  conjunction 
with  rolled  steel  joists,  or  heavier  section 
ribbed  steel  bars  alone. 

Dawnay  Floors.  — The  reinforcement 
consists  of  rolled  steel  joists  at  16-in.  in- 
tervals, or  square  bars  at  12-in.  intervals, 
laid  between  other  joists. 

Ellis  Pipes. — Round  steel  rods  reinforce 
sections  as  short  as  2  ft.  or  3  ft.,  there  being 
an  ogee  joint  to  each  section. 

Hodkin-Jones  Floors. — These  are  re- 
inforced with  corrugated  bars  placed  on 
edge  and  resting  on  steel  girders  through 
the  medium  of  a  bent  and  slotted  plate. 

Homan  Floors. — These  are  of  many 
types.  In  one,  the  webs  of  rolled  steel 
joists  are  pierced  to  allow  of  the  passage  of 
round  bars  which  project  into  adjoining 
slabs ;  and  in  another,  the  reinforcement 
takes  the  form  of  a  T-bar  with  corrugated 
web. 

Koenen  Floors. — Slabs  are  haunched 
at  each  side  near  the  supporting  piers  or 
joists,  the  reinforcement  of  round  bars 
lying  flat  between  the  haunches  and  being 
turned  up  at  the  ends  so  as  to  be  securely 
anchored. 

Lindsay  Floors. — Slabs  may  be  bounded 
by  girders  on  two  opposite  sides,  and  by 
rolled  steel  joists  on  the  other  two.  Pairs 
of  round  bars  cross,  in  the  vertical  plane 
in  the  middle  of  the  slab,  one  bar  passing 
over,  and  the  other  under,  the  next 
joist. 

Potter  Floors. — The  tensional  reinforce- 
ment consists  of  reinforced  corrugated  rods. 
When  the  span  exceeds  12  ft.,  rolled  steel 
joists  are  introduced. 

Ridley  Cammell  Floors,  Columns,  etc. 
—The  beam  reinforcement  is  a  trough  of 
dovetail  corrugated  steel  sheeting  and  angle 
bars,  the  slab  reinforcement  consisting  of 
corrugated  sheets.  Concrete  is  applied  to 
both  sides  of  the  reinforcement,  but  center- 
ing, etc.,  is  unnecessary,  it  is  claimed. 
Column  reinforcement  is  a  combination  of 
the  corrugated  sheet  and  bars  of  various 
sections  to  form  a  cage.  For  walls,  also, 
the  sheeting  is  used,  studs,  if  required,  being 
provided  by  bars. 

Somerville  Floors.  —  Reinforced  flat- 
bottomed  hollow  blocks  with  curved  tops 
are  supported  by  the  flanges  of  rolled  steel 
joists,  a  top  layer  of  concrete  completing 


228 


REINFORCED    CONCRETE 


the  floor.  In  another  type,  the  blocks, 
much  as  before,  are  supported  by  reinforced 
concrete  beams,  which,  when  the  upper 
layer  of  concrete  is  in  place,  become  tee- 
beams. 

Wells  Beams,  Floors,  etc. — The  ten- 
sional  reinforcement  of  a  beam  consists  of 
twin  bars  connected  by  a  web,  the  bars  being 
placed  on  edge  when  several  are  employed. 
Some  of  the  bars  may  be  bent  up  at  the 
ends,  and,  in  addition,  there  may  be  straight 
bars  to  take  part  of  the  compression.  The 
shear  reinforcement  consists  of  vertical 
stirrups,  these  being  known  as  "  hangers  " 
when  they  connect  the  upper  and  lower 
reinforcements,  and  as  "  bonders "  when 
projecting  downwards  from  the  compression 
bars  to  the  neutral  axis.  Slabs  connect 
main  and  secondary  beams  in  the  usual 
way  to  form  floors.  Rectangular  columns 
of  moderate  size  have  a  round  bar  near 
each  angle,  the  transverse  ties  being  round 
links  with  a  hook-and-eye  joint.  In  larger 


columns,  the  links  are  connected  to  the 
vertical  reinforcements. 

Wilkinson  Floors. — Floor  slabs  have 
round  bar  reinforcements  in  both  directions, 
with  their  ends  bent  over  for  more  secure 
anchorage. 

Williams  System. — Small  beams  are  rein- 
forced near  the  lower  surface  with  I-section 
bars  which  take  the  tension,  while  the 
shear  is  taken  by  vertical  round  bars  split 
at  the  ends.  Larger  beams  have  I-section 
compression  bars,  and  still  heavier  ones  are 
reinforced  with  railway  metals,  and  with 
diagonal  bars  so  attached  to  the  reinforce- 
ments that  the  whole  is  self-supporting  in 
the  mould.  The  slabs  connecting  floor 
beams  contain  small  rolled  joists  placed, 
parallel  and  laced  together  with  hoop  steel 
passing  above  and  below  the  small  joists 
alternately.  Joists  with  flat  steel  bars, 
riveted  to  them  are  used  in  piles  constructed 
on  the  Williams  system,  the  end  of  the 
joist  being  shaped  to  act  as  a  driving  point. 


The  Architectural  and  Surface  Treat- 
ment of  Reinforced  Concrete 


ALTHOUGH  European  and  American  archi- 
tects are  now  using  reinforced  concrete 
•extensively,  it  must  be  admitted  that,  as  far 
as  buildings  of  any  architectural  pretension 
are  concerned,  it  is  almost  entirely  used  in 
hidden  situations,  such  as  foundations,  floors, 
beams,  columns,  etc.,  or  perhaps  as  a  backing 
to  the  external  walling,  either  brickwork  or 
masonry,  and  its  use  rarely  modifies  the 
external  expression  of  the  building,  except 
in  the  case  perhaps  of  commercial  or  indus- 
trial architecture.  As  an  example  of  rein- 
forced concrete  building  clothed  by  masonry, 
attention  may  be  directed  to  Mr.  W.  Aubrey 
Thomas's  Koyal  Liver  Building,  Liverpool 
(see  pages  329  to  333),  in  which  the  external 
walls  are  of  granite  not  more  than  14  in. 
tlu'ck.  In  this  building  the  great  difficulties, 
from  an  architectural  point  of  view,  of  deal- 
ing with  an  erection  of  its  height  have  been 
very  successfully  overcome. 

The  statement  is  frequently  made  that 
architects  do  not  like  reinforced  concrete, 
and  to  a  certain  extent  this  is  true.  Why  ? 
Probably  the  reason  is  to  be  found  in  the  fact 
that  concrete  has  little  or  no  inherent  beauty. 
Building  stones,  slates,  bricks,  and  tiles  have 
a  beauty  of  their  own,  either  of  colour  or  of 
texture,  or  both,  irrespective  altogether  of 
the  design  of  which  they  form  a  part,  and 
this  beauty  is  generally  increased  by  the 
action  of  the  weather.  This  is  also  true  of 
timber,  with  its  infinite  variation  of  graining 
and  colour.  This  being  so,  architects  have 
not  in  the  past  considered  reinforced  con- 
crete as  likely  to  become  a  serious  competitor 
with  traditional  materials  for  external  walls. 

Many  who  visited  the  Paris  1900  Exhibi- 
tion probably  had  reinforced  concrete 
brought  to  their  notice  for  the  first  time. 
This  exhibition  had  undoubtedly  much  in- 
fluence on  the  recognition  of  reinforced  con- 
crete in  England  and  throughout  Europe 
generally,  and  later  exhibitions  have  still 
further  advertised  its  claims.  The  larger 
class  of  exhibition  buildings  are  admirably 
adapted  to  advance  the  architectural  claims 
of  the  "new"  material,  as  in  their  design  a 


degree  of  freedom  may  be  permissible  that 
would  not  be  allowed  in  more  enduring  erec- 
tions. This  has  been,  to  some  extent,  taken 
advantage  of,  but  not  so  largely  as  might 
be  wished. 

In  considering  the  question  of  reinforced 
concrete  and  the  influence  it  is  likely  to 
have  on  architectural  design  in  the  future,  it 
must  be  admitted  that  a  really  satisfactory 
treatment  typical  of  the  material  has  yet 
to  be  found.  This  will  not  be  achieved  by 
any  one  man,  but  will  only  be  brought  about 
gradually,  in  the  course  of  time,  as  the 
material  comes  into  more  general  use  in 
architecture,  each  architect  adding  his  small 
contribution  towards  the  solution  of  the 
problem.  In  other  words,  it  will  be  a  growth, 
just  as  all  past  developments  have  been. 

The  guiding  principle  of  the  civil  engineer 
is  to  use  as  little  material  as  possible,  and 
this  explains  why  he  has  so  readily  adopted 
reinforced  concrete,  for  that  is  precisely  the 
principle  on  which  it  is  designed.  Whether 
it  is  an  altogether  good  principle  from  an 
architectural  point  of  view  is  another  matter, 
since  the  architect  in  all  monumental  work 
uses  mass  as  an  element  of  expression,  but 
it  must  be  remembered  that  during  the 
evolution  of  Gothic  architecture  the  loads 
were  carried  on  comparatively  small  sup- 
ports, and  these  were  reduced  more  and 
more  as  the  style  progressed,  the  reason 
being  most  probably  purely  aesthetic.  A 
similar  tendency  to  reduce  the  amount  of 
material  employed  to  a  minimum  is  seen  in 
reinforced  concrete  design  ;  but  in  this  case 
it  is  for  frankly  economical  reasons.  Instead 
of  continuous  walling,  the  weight  of  the 
building  is  carried  down  to  the  foundations 
on  piers,  external  and  internal,  the  floors 
resting  on  internal  beams  or  on  wall  lintels 
spanning  from  pier  to  pier,  and  the  exterior 
skeleton  being  united  by  thin  panel  walls 
which  carry  only  their  own  weight  and  are 
used  merely  to  keep  out  the  weather.  This 
thinness  of  the  external  walling  is  one  of 
the  drawbacks  of  reinforced  concrete  from 
an  architectural  standpoint,  as,  owing  to  the 


229 


Fig.  424. — The  Upper  Stories  of  a  Reinforced  Concrete  Warehouse  at    Cologne. 

Front  View 


Fig.   425.— End   View  of  Reinforced  Concrete  Warehouse  at  Cologne 


230 


ARCHITECTURAL    AND    SURFACE   TREATMENT 


231 


difficulty  of  obtaining  effective  shadows,  a 
flat  treatment  seems  almost  inevitable,  it 
being  impossible  to  get  any  depth  to  the 
window  reveals  save  by  subsidiary  piers  or 
other  similar  expedients.  Tor  this  and 
other  reasons  it  does  not  appear  likely  that 
the  traditional  materials  need  fear  any  rival 
for  some  long  time  to  come  in  the  largest 


Fi>.  426 


Fig.  427 


and  most  important  architectural  work.  It 
seems  probable  that  in  work  of  this  class  its 
greatest  opportunities  of  affecting  design  will 
be  in  the  construction  of  domes,  vaulting, 
ceiling  lights,  roofs,  and  similar  works,  for 
which  the  material  is  so  admirably  suited 
that  it  is  hard  to  conceive  any  other  method 
of  building  by  which  such  good  results  could 
be  so  easily  attained. 

Inasmuch  as  there  is  a  temptation  to 
overrate  the  position  of  reinforced  con- 
crete and  the  influence  it  is  likely  to 
have  over  architectural  design  in  the 
future,  it  may  be  advisable  to  consider 
its  possibilities  and  limitations  as  a 
constructional  material.  It  is  frequently 
spoken  of  as  though  its  use  enabled 
larger  spaces  to  be  covered  than  was 
ever  before  possible  ;  but  this  is  not  the 
case,  and,  although  we  have  now  in 
engineering  constructions  some  remark- 
able examples  of  the  distances  that  can 
be  spanned  by  the  scientific  use  of  the 
material,  the  longest  span  at  present  is 
to  be  found  in  the  Grafton  bridge  con- 
structed by  the  city  authorities  of  Auck- 
land, New  Zealand.  The  bridge  is  960  ft. 
long  and  40  ft.  wide,  and  the  main  arch 


has  a  span  of  320  ft.,  although  this  will 
be  exceeded  by  the  Albert  Bells  bridge  in 
Rome.  It  is  practically  certain  that  it  will 
always  be  possible  by  means  of  steel  to  span 
larger  openings  than  would  be  the  case  were 
reinforced  concrete  employed.  But  steel, 
when  used  in  buildings,  must  be  cased,  as, 
even  if  its  successful  treatment  from  an 
architectural  point  of  view 
did  not  render  this  necessary, 
its  lack  of  fire-resisting  quali- 
ties would  make  it  impera- 
tive. This  casing,  while  pro- 
tecting the  steel  from  fire,  is 
often  a  source  of  positive 
weakness  so  far  as  the  action 
of  the  weather  is  concerned, 
as  it  prevents  the  periodical 
painting  of  the  steel.  This 
suggests  another  great  advan- 
tage possessed  by  reinforced 
concrete,  namely,  its  reduc- 
tion of  maintenance  expenses 
to  an  absolute  minimum. 
This  and  the  rapidity  with 
which  reinforced  concrete  con- 
structions can  be  erected  are 
most  important  factors  when 
comparing  the  cost  of  erec- 
tions by  the  two  methods. 

Again,  it  must  be  admitted  that  a  rein- 
forced concrete  beam  is  a  more  truthful, 
architecturally  and  scientifically  defensible 
method  of  construction  than  a  rolled-steel 
joist  cased  ;  and  if  the  former  is  used  in 
connection  with  a  continuous  floor  slab  the 
beams  are  all  converted  into  T-beams,  which 


Fig.  428 


Figs.  426  to  428.— Verti- 
cal   and   Horizontal   Sec- 
tions of  Small  Hall  with 
Arched  Ribs 


Fig.  429. — Interior  View  of  Hall  with  Arched 
Ribs 

adds  greatly  to  their  strength,  the  whole 
being  in  monolithic  connection  with  the  build- 
ing generally.  With  reinforced  concrete,  as 
with  steel,  where  large  spaces  have  to  be 


232 


REINFORCED    CONCRETE 


spanned,  depth  in  the  beam  is  indispensable  ; 
but  if  the  architectural  arrangements  of  the 
interior  permit,  the  beams  may  be  most 
economically  constructed  in  concrete  as 
arched  beams,  so  reinforced  that  no  thrust 
is  transmitted  to  the  supporting  piers. 

Some   good   examples   of   arched    beams 
carrying  very  heavy  loads  are  to  be  found 


Fii.  430 


'o 


Fig.    431 


in  the  Koyal  Liver  building  previously  men- 
tioned, and  in  the  Wesleyan  Central  Build- 
ings, Westminster  (see  pp.  337  to  346). 

With  regard  to  stanchions  and  columns, 
it  will  be  found  that  these,  generally,  when 
constructed  in  reinforced  concrete,  will  work 
out  to  about  the  same  or  slightly  smaller 
sectional  area  as  would  have  been  the  case 
had  a  steel  stanchion  been  designed  to  carry 
the  same  load  and  then  cased  with  some 
fire-resisting  material  in  the  usual  manner, 
so  that,  as  far  as  proportion  is  concerned, 
the  position  is  practically  what  it  was  before. 
Yet,  here  again,  the  advantages  claimed  for 
reinforced  concrete,  when  used  in  the  con- 
struction of  beams,  could  with  equal  force 
be  claimed  for  the  material  when  employed 
in  supporting  them.  In  connection  with 
the  above  remark  on  column  proportions, 
the  two  following  examples  may  prove 
interesting.  In  the  first  case,  the  length  of 
the  column  was  20  ft.  and  the  load  to  be 
carried  200  tons.  By  designing  in  reinforced 
concrete  the  column  worked  out  to  22£  in. 
square.  By  selecting  a  steel  stanchion  and 
allowing  2  in.  all  round  for  fire  casing  the 
result  was  18  in.  by  17J  in.  In  the  second 
case,  the  length  of  the  column  was  12  ft., 


and  the  load  to  be  carried  40  tons ;  the 
reinforced  concrete  column  worked  out  to 
10  in.  square,  and  the  steel  stanchion,  allow- 
ing 2  in.  all  round  as  before,  to  12  in.  by 
10  in. 

It  must  not  be  forgotten  that  reinforced 
concrete  is  introducing  no  such  revolutionary 
or  very  great  change  in  architectural  ex- 
pression as  that  which  occurred 
when  the  arch  superseded 
the  lintel.  It  is  interesting 
to  find  Prof.  Beresford  Pite,  in 
the  course  of  his  Carpenters' 
Hall  lecture  on  "  What  is 
Artistic  Craftsmanship  ?  "  say- 
ing :  "  If  the  Greeks  had  had 
the  use  of  steel  girders  or 
ferro-concrete  material  they 
would  have  been  delighted  to 
find  that  they  could  span  the 
whole  of  a  big  front  with  'a 
beam  such  as  has  been  em- 
ployed in  the  new  Post  Office 
building  in  Newgate  Street." 
It  seems  practically  certain 
that  the  Greeks  had  a  know- 
ledge of  the  arch,  and  that 
they  preferred  and  deliber- 
ately selected  the  lintel  for 
covering  their  openings,  in  spite  of  the 
former  rendering  larger  openings  possible 
but  it  does  not  follow  that  they  would  not 
have  welcomed  a  stronger  beam,  although 
their  desire  to  span  larger  openings  was  not 
emphatic  enough  to  force  them  to  adopt  the 


Fig.  432 


Figs.  430  to  432.— 
Vertical  and  Hori- 
zontal Sections  of 
Small  Hall  with 
Barrel  Roof 


Fig.   433. — Interior  View  of   Hall  with    Barrel 
Roof 


arch.  Allowing  that  the  physical  properties 
of  marble  were  the  main  factors  that  fixed 
the  general  proportions  of  Greek  architecture, 
at  any  rate  as  far  as  the  spacing  of  the 
columns  was  concerned,  it  must  be  admitted 


ARCHITECTURAL   AND    SURFACE   TREATMENT 


233 


that  no  such  definite  proportions  could  be 
expected  by  using  reinforced  concrete.  A 
marble  or  masonry  beam  of  a  certain  sectional 
•area  is  capable  of  doing  a  definite  amount  of 
work  ;  but  in  the  case  of  reinforced  concrete 
the  strength  of  the  beam  depends  so  largely 
on  the  reinforcement  which  is  hidden,  that 
•no  such  definiteness  is  possible,  although 
there  is  a  certain  proportion  between  the 
•concrete  and  the  steel  in  a  beam  which  may 
be  said  to  be  economically  ideal. 

The  following  quotation  from  Garbett's 
able  book  on  architectural  design  is  very 
suggestive :  "A  new  style  requires  the 
generalised  imitation  of  Nature  and  of  many 


Fig.  434 


Fig.  435 


previous  styles  ;  and  a  new  system  requires, 
in  addition  to  this  (as  Prof.  Whewell  has 
remarked)  the  binding  of  all  together  by  a 
new  principle  of  unity,  clearly  understood, 
agreed  upon  and  kept  constantly  in  view. 
Constructive  statics  afford  three  such  prin- 
ciples— the  depressile,  the  compressile,  and 
the  tensile  methods — the  beam,  the  arch,  the 
truss,  of  which  the  two  former  have  been 
made  the  bases  of  past  systems  ;  the  third 
is  ours,  to  be  used  in  the  same  manner."  In 
reinforced  concrete,  the  binding  principle  of 
the  truss  is  introduced  into  beams  by  giving 
them  the  rods  to  take  the  tensile  strain,  thus 
economising  material  and  rendering  larger 
spans  possible. 

It  is  to  some  of  the  larger  manufacturing 
buildings  in  which  reinforced  concrete  has 


been  employed  where  the  mask  walls  are 
uncased,  or  to  engineering  works,  that  we 
must  turn  to  find  treatments  which,  although 
guided  by  economical  and  strictly  construc- 
tional reasons,  may  be  said  to  be  most 
typical  of  the  nature  of  the  material. 
Obviously,  buildings  that  are  to  be  hidden 
by  masonry  are  more  interesting  from  a  con- 
structional point  of  view  while  in  the  skeleton 
state  than  after  being  clothed.  The  general 
resemblance  between  their  proportions  and 
those  of  steel  framework  should  be  noted, 
but  we  must  avoid  letting  the  material  in 
question  have  too  arbitrary  a  sway,  as  we 
find  that  from  the  earliest  times,  and  during 
the  most  flourishing 
periods  of  architec- 
ture, design  in  one 
material  has  fre- 
quently been  greatly 
influenced  by  design 
in  other  and  some- 
times very  dissimilar 
materials.  As  an  ex- 
ample, we  have  only 
to  take  the  Greek 
Doric  order,  with 
its  triglyphs,  guttse, 
and  mutules,  which, 
although  executed  in 
marble,  are  obviously 
founded  on  timber 
construction. 

Taking,  then,  an 
ordinary  reinforced 
concrete  warehouse, 
the  piers  taking  the 
weights  will  probably 
be  found  to  show  on 

both  the  inside  and  outside ;  the  floors,  where 
they  come  to  the  outer  wall,  will  be  carried 
by  wall  lintels  or  beams,  which  also  show ; 
the  roof  will  be  flat ;  the  exterior  mask  walls, 
or  panelling,  possibly  about  5  in.  or  6  in. 
thick,  and  the  window  openings  fitted  with 
steel  casements.  The  principal  defect  (leav- 
ing out  the  question  of  surface  finish)  is 
generally  the  lack  of  proportion ;  other 
defects  are  flatness  and  want  of  shadows. 
The  last  mentioned  might  be  partly  remedied 
by  keeping  the  faces  of  the  wall  piers,  wall 
lintels,  and  curtain  walls  flush,  or  almost 
flush,  on  the  inside,  and  by  making  the  piers 
of  such  a  shape  on  plan  that  they  will  fit 
their  position  in  the  general  design.  This 
can  frequently  be  done  without  in  any  way 
increasing  their  sectional  area.  If  the  main 


Fig.  436 


Figs.  434  to  436. — Vertical 

and  Horizontal  Sections  of 

Small    Hall    with    Modified 

Barrel  Roof 


234 


REINFORCED    CONCRETE 


piers  are  fairly  widely  spaced,  two  subsidiary 
piers  may  in  some  cases  be  with  advantage 
introduced  between  them,  and  if  the  inter- 
vening windows  occupy  the  whole  width 
between  the  subsidiary  piers,  more  satis- 
factory reveals  can  be  obtained.  The  pro- 
jection of  the  cornice  could  generally  be 


Fig.  437 


rectangular,  having  small  projections  at 
each  end  of  the  main  block.  The  work  has 
been  carried  out  entirely  in  reinforced  con- 
crete subsequently  treated  with  yellow 
roughcast,  which,  being  subdued  in  tone, 
gives  a  really  pleasing  colour  effect.  For 
the  roof  rough-dressed  local  slates  of  a  green 
tint  have  been  em- 
ployed in  an  attrac- 
tive manner.  The 
general  massing  of  the 
building  is  good,  and 
the  effect  of  breaking 
up  the  roof  surfaces  is 
excellent.  It  is  in  the 
massing  of  the  roof 
that  the  chief  charm 
of  the  building  lies ; 
the  bold  lines  of  the 
gables  and  slate-hung 
dormers  are  effective, 
and  the  way  in  which 
the  roof  surfaces  rise 
one  above  the  other 
gives  a  charm  to  the 
design.  With  regard 
to  the  method  in 
which  the  slates  have 
been  used  there 


Fig.  439 


Figs.  437  to  439.— Ver- 
tical and  Horizontal 
Sections  of  Small  Hall 
with  Pierced  Arched 
Ribs 


Fig.  438 


greatly  increased  with  advantage  to  the 
building.  Another  way  of  obtaining  good 
horizontal  shadows  is  by  means  of  bal- 
conies, where  the  character  of  the  building 
renders  them  desirable.  They  can  be  formed 
by  continuing  the  floor  slab  through  the  wall, 
the  reinforcing  rods  being  continued  and 
turned  up  in  the  parapet  or  balustrade,  the 
whole  being  in  monolithic  connection.  Where 
subsidiary  piers  are  inadmissible,  and  very 
flat  reveals  are  unavoidable,  then  probably 
casements  with  broad  frames  round,  in  what- 
ever material  they  might  be  constructed, 
would  give  the  best  results,  just  as  in  a  flat 
Georgian  treatment  we  have  the  broad 
frames  of  the  sashes  with  a  mould  round  set 
nearly  flush  with  the  external  face  of  the 
brickwork. 

There  is  an  interesting  example  of  rein- 
forced concrete  construction  on  the  left  bank 
of  the  Khine  at  Cologne  ;  it  is  a  warehouse, 
two  views  of  which  are  shown  by  Figs.  424 
and  425,  p.  220.  In  plan  the  building  is 


workmen.      In    the 


is 

much  to  learn  from 
the  Germans,  who  are 
apparently  far  more 
skilled  in  the  use  of 
slates  than  are  English 
whole  roof  a  visitor 


could  not  discover  a  single  instance  of  the 


Fig.  440. — Interior  View  of  Hall  with  Pierced 
Arched  Roof 

use  of  lead.     All  hips,  valleys,  gutters,  etc., 
are  worked  with  the  slates  themselves.    This 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


235 


masterly  use  of  the  slates,  together  with 
the  well-designed  sprocketing,  has  the  effect 
of  softening  down  the  roof  lines  to  a  remark- 
able degree.  Taken  as  a  whole,  the  design 


Fig.  441. — Interior  View  of  Hall  with  Arched  Ribs  and  Vertical 

Columns 


is  most  instructive,  and  helps  to  show  what 
vast  possibilities  there  are  for  fine  archi- 
tectural treatment  of  factory  buildings, 
warehouses,  etc.,  in  reinforced  concrete. 

Turning  to  engineering  works,  particu- 
larly bridges,  some  very  successful  and 
architecturally  suggestive  designs  have  been 
executed  in  reinforced  concrete.  In  bridges 
that  are  typical  of  reinforced  concrete  bridge 
design,  there  are  two  arched  ribs,  or,  if  the 
bridge  is  wide,  a  series  of  arched  ribs,  with 
vertical  columns  resting  on  them,  giving  the 
effect  of  open  spandrels,  and  supporting 
longitudinal  and  transverse  beams,  which  in 
turn  carry  the  road  decking.  Some  very 
light  and  graceful  effects  are  obtained  by 
working  on  these  lines,  and  the  treatment 
also  affords  suggestions  which  should  prove 
useful  in  roof  designs. 

In  the  designs  which  will  now  be  referred 
to — designs  that  have  been  prepared  with  a 
view  to  their  suitability  for  execution  in  rein- 
forced concrete — the  endeavour  has  been  to 
show  how  some  of  the  suggestions  previ- 
ously made  might  be  allowed  to  have  their 
influence  ;  but,  of  course,  the  illustrations 
must  be  taken  as  merely  showing  proposed 
treatments,  and  not  as  working  drawings. 
Figs.  426  to  429  show  a  cross  section, 
plan,  the  longitudinal  section  of  two  bays, 
and  perspective  outline  through  a  building 


that  would  be  suitable  for  a  small  parish 
hall.  The  springing  being  so  low,  the  only 
form  of  wooden  principal  that  could  have 
been  used  would  have  been  of  the  hammer- 
beam  type,  and,  the 
span  being  40  ft.,  this 
would  have  made  heavy 
buttresses  a  necessity. 
The  proposal  is  to  in- 
sert reinforced  concrete 
arched  ribs,  carried  down 
right  on  to  the  founda- 
tions— the  pier  being  a 
feature  inside  as  well  as 
outside — and  designed  so 
as  to  exert  no  outward 
thrust,  connected  by  con- 
tinuous reinforced  con- 
crete roof  and  ceiling 
slabs,  the  former  of 
which  come  down  on  to 
reinforced  concrete  beams 
between  the  arched  ribs 
at  eaves  level,  and  all 
being  in  monolithic  con- 
nection. Below  the  eaves 


the  outer  walls,  which  are  merely  mask  walls 
and  carry  no  weight,  could  be  constructed 
as  desired  according  to  the  character  of  the 
external  design  and  the  funds  available. 
No  reinforced  concrete  purlins  have  been 


Fig.   442. — Part    Longitudinal    Section    of   Hall 
with  Arched  Ribs  and  Vertical  Columns 

shown,  as  the  idea  was  by  getting  away  from 
the  effect  of  timber  construction  to  produce 
something  more  typical  of  the  new  material ; 
but  if  inserted  they  would,  by  making  a 


236 


REINFORCED    CONCRETE 


thinner  roof  slab  possible,  cheapen  the  cost. 
Again,  if  the  arched  ribs  are  looked  upon  as 
only  taking  the  place  of  timber  roof  trusses, 
the  rest  of  the  roof  could  be  finished  with 
wooden  purlins,  rafters,  etc.,  in  the  ordinary 
way,  and  would  be  still  cheaper,  though  not 
nearly  so  satisfactory. 

By  means  of  sections,  plan,  and  outline 
perspective,  Figs.  420  to  433  show  another 
small  hall,  but  this  time  with  barrel  roof, 
the  arched  ribs  showing  below  same,  spring- 
ing from  piers  and  being  connected  at  eaves 
level  by  reinforced  concrete  beam?,  and 
having  continuous  reinforced  concrete  roof 
slabbing  and  barrel  vault,  the  external  walls 
only  keeping  out  the  weather  as  before. 

The  plan  and  sections  in  Figs.  434  to  436 
show  a  variation  of  a  barrel  roof,  with  flat, 
panelled  ceilings  on  each  side.  In  this 
•design  the  whole  is  intended  to  be  executed 
in  reinforced  concrete,  except  the  mask  walls, 
which  are  in  brick.  The  arched  rib  principle 
is  abandoned,  and  the  tie-beam  adopted,  and 
although  this  represents  a  type  well  adapted 
for  construction  in  reinforced  concrete,  it 
yet  does  not  seem  so  typical  of  the  material 
as  the  others,  but  rather  more  suggestive 
of  timber. 

Figs.  437  to  440,  with  two  sections,  plan, 
and  perspective  sketch,  show  a  rather  more 
elaborate  roof,  with  pierced  arched  ribs  con- 


Fig.   443. — Cross  Section  of  Hall  with  Arched  Ribs  and 
Vertical  Columns 


nected  by  reinforced  concrete  beams  at 
eaves  level,  moulded  to  form  interior  and 
exterior  frieze  and  cornice,  with  roof  and 
ceiling  slabs,  etc.,  all  as  before ;  but,  in 


addition,  the  pierced  ribs  are  connected  by 
arched  purlins,  and  the  piers  inside  and  out 
have  caps  and  bases. 

A  type  of  roof  founded  on  reinforced 
bridge  design  is  shown  in  perspective,  part 
longitudinal  section,  and  cross  section,  by 
Figs.  441  to  443.  There  are  arched  ribs  at 
intervals,  as  in  bridges, with  vertical  columns, 
supporting  longitudinal  and  transverse 
beams,  which  in  turn  support  the  roof  slab. 
This  corresponds  to  the  bridge  decking,  save 
that  it  is  on  the  slope,  and  is  treated  just 
as  a  reinforced  concrete  floor  would  be 
treated,  with  main  beams  over  the  arched 
ribs,  and  secondary  beams  spanning  from 
main  beam  to  main  beam.  A  continuous 
skylight  is  shown  ;  the  beams  supporting  it 
are  connected  by  a  series  of  arched  ribs  with 
open  spandrels  springing  from  main  arched 
rib  to  main  arched  rib.  The  rest  of  the 
design  is  sufficiently  explained  by  the  draw- 
ings. This  represents  a  form  of  roof  design 
capable  of  great  development,  and  perhaps 
more  typical  of  reinforced  concrete  than 
any  of  the  others. 

Figs.  444  to  448  show  a  design  for  an  exhi- 
bition hall,  which  is  illustrated  by  means  of  a 
longitudinal  section  showing  two  bays  out  of 
three,  plan  of  same,  outline  perspective,  cross 
section,  and  part  plan  of  ceiling.  It  was  sug- 
gested by  the  Renommee  Hall,  Liege, 
designed  by  M.  Paul  Jaspar. 
As  shown,  the  main  hall 
is  45  ft.  wide,  with  side 
aisles  and  galleries  all  round, 
16  ft.  6  in.  wide.  The  main 
hall  is  divided  into  three 
bays  in  the  length,  each 
being  45  ft.  square  and 
having  a  flat  saucer  dome, 
with  ceiling  light  continuing 
the  same  curve,  over.  The 
saucer  domes  are  of  such 
a  radius  that  if  the  curve 
is  continued  the  diameter 
of  the  complete  semicircle 
equals  the  diagonal  of  the 
bays.  The  main  piers  are 
connected  longitudinally 
and  transversely  under  the 
saucer  domes  by  semi- 
circular arches.  The  saucer 
domes  are  ribbed  and  pan- 
elled, exhibiting  the  real  construction,  and 
the  ribs,  if  continued  in  the  same  curve,  abut 
either  upon  the  semicircular  arches  men- 
tioned or  upon  the  main  piers,  but  below  the 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


237 


level  of  the  gallery  ceiling,  which  is  also  the 
level  of  the  flat  ceiling  of  the  hall.  The 
panelling  between  the  ribs  is  omitted  ;  in 
other  words,  we  have  what  might  be  called 


open  ribs  we  see  the  flat  ceiling  of  the  hall, 
which  is,  of  course,  of  very  small  area,  as 
nearly  the  whole  area  of  the  main  hall  ceiling 
is  taken  up  by  the  domes.  This  is  a  treat- 


Fig.   4-14. — Part  Longitudinal  Section  of  Exhibition  Hall  with  Three  Flat  Saucer  Domes 


Fig.   445.— Part  Plan  of  Exhibition  Hall 

open  pendentives,  which  give  a  fan-like  effect  ment    which    contains    many    suggestions 

to  the  ribs  springing  out  of  the  main  piers  capable  of  further  development.     The  whole 

and  continuing  those  of  and  supporting  the  of  the  work  shown  is  intended  to  be  executed 

saucer  domes.     By  looking  up  between  these  in  reinforced  concrete. 


238 


REINFORCED    CONCRETE 


A  design  for  a  fa§ade  suitable  for  a  club  or 
similar  building,  to  be  erected  in  reinforced 
concrete,  is  shown  in  Figs.  449  to  452.  The 
desire  has  been  to  produce  a  design  as  typical 
of  the  material  as  possible,  and  to  base  the 


Fig.  446.— Interior  View  of  Exhibition  Hall 


the  reinforcing  rods  in  the  flat  roof  turned 
up  into  it  as  shown,  the  whole  being  in 
monolithic  connection.  In  order  to  obtain 
the  effect  of  a  frieze  to  the  building,  the 
filling-in  walls  to  the  top  story  have  been 
brought  out  flush  with  the 
front  of  the  piers,  and  the 
floor  slab  at  third-floor  level 
being  brought  beyond  the  face 
of  the  piers  while  the  wall 
lintels  are  flush,  the  effect 
of  a  narrow  architrave  is 
obtained.  The  remarks  that 
applied  to  the  cornice  apply 
also  to  the  balcony  at  first- 
floor  level.  This  has  been 
shown  with  a  panelled  front 
in  the  same  material,  although, 
of  course,  a  wrought-iron  rail- 
ing would  be  just  as  suitable 
a  finish  to  a  reinforced  con- 
crete balcony  as  to  one  in 
stone.  In  order  to  get  some 
effective  vertical  shadows,  the 
front  has  been  treated  with  a 
feature  at  each  end  and  a 


architectural  treatment  throughout  on  the 
structural  features  of  the  building.  The 
cornice  has  been  given  a  good  projection,  the 
blocks  under  it  being  merely  the  floor  beams 


recessed  cen- 
tral portion. 
The  windows 
throughout  are 


Fig.   447. — Cross  Section  of  Exhibition  Hall  through  one 
of  the  Domes 


Fig.  448.— Part  Plan  of  Ceiling 
of  Exhibition   Hall 


continued  through  the  wall,  with  brackets 
under.  The  necessity  for  the  blocking  being 
at  the  back  of  the  cornice  to  weigh  it  down, 
as  in  masonry  construction,  having  dis- 
appeared, it  has  been  brought  forward,  and 


steel  casements ;  those  to  second  floor,  hav- 
ing no  subsidiary  piers  to  give  depth  to  the 
reveals,  have  been  given  broad  frames  which 
lap  over  the  concrete.  The  wide  ground- 
floor  windows  to  the  end  features  are  kept 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


239 


nearly  flush  with  the  back  of  the  piers,  so 
that  good  reveals  are  obtained.  The  rest 
of  the  ground-floor  windows  are  also  kept 
back,  but  small  subsidiary  piers  are  intro- 
duced, and  the  filling  in  is  on  the  cant.  The 
first-  and  second-floor  windows  to  the  end 
features  are  treated  as  flat  bays,  the  filling 
in  sloping  from  the  small  subsidiary  piers 
to  the  back  of  the  main  piers. 

The  remaining  first-floor  windows  should 
have  small  subsidiary  piers,  which  are 
treated  externally  as  pilasters,  with  brackets 
and  hoods  ovelr.  The  hoods  are  brought 


out  in  a  curve  from  the  face  of  the  wall,  as 
is  often  done  in  roughcast,  this  seeming  to 
suggest  a  plastic  material. 

The  filling-in,  or  mask,  walls  have  been 
designed  so  as  to  suggest  panelling,  and  the 
face  of  the  wall  lintels  to  the  second  floor 
has  been  kept  back  from  the  face  of  the 
piers  and  flush  with  the  filling,  so  as  to  give 
more  pleasing  proportions. 

The  exterior  surface  is  intended  to  be 
finished  without  any  applied  ornament. 
The  exterior  of  piers  and  beams  would  be 
finished  with  a  smooth  surface,  obtained  by 


-e 
U 


Fig.  451  Fig.  452 

Figs.  451  and  452. — Detail  of  Reinforced  Concrete  Facade  in  Elevation 
and  Vertical  Section 


240 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


241 


ubbing  down  the  surfaces  after  the  removal 
if  the   moulds   and  floating  them   with   a 
hin  wash  of  cement  and  sand  grout.      The 
ixterior    of    panels    and    other    sunk    sur- 
ges would  be  treated  by  washing  out  the 
outer  film  of  cement  so  as  to   expose  the 
particles  of  stone  used  as  matrix  and  aggre- 
;ate. 

As  forming  a  contrast  to  the  illustrations 
above  referred  to,  attention  will  now  be 
directed  to  Figs.  453  to  457,  which,  while 
scarcely  typifying,  undoubtedly  suggest  the 
place  taken  in  the  past  by  reinforced  con- 
crete considered  purely  architecturally.  The 


SURFACE    TREATMENT 

The  reason  offered  by  architects  why 
reinforced  concrete  is  nearly  always  covered 
up  is  its  alleged  lack  of  inherent  beauty 
as  a  building  material.  Fortunately,  the 
general  adoption,  some  time  in  the  future, 
of  more  artistic  methods  of  finishing  will 
remove  the  reproach.  Clearly,  until  rein- 
forced concrete  is  boldly  exposed  it  will 
never  be  able  to  exert  its  legitimate  influ- 
ence over  external  design,  although,  even 
as  now  used,  it  has  had  a  certain  influence 
on  design,  particularly  in  commercial  archi- 
tecture. Let  it  be  remembered  that  a  build- 


Fig.  453. — Small   Hall  with  Mask  Walls  of  Brick  and  Principals  and  Roof  of  Reinforced 

Concrete 


illustrations  show  a  small  public  hall  or 
parish  hall  apparently  built  of  traditional 
materials,  the  exterior  walls  being  in  local 
bricks  with  red  facings,  and  the  interior 
walls  being  in  Fletton  bricks.  These  walls 
are  merely  mask  walls,  however,  carry- 
ing no  weight,  and  they  could  just  as 
well,  apart  from  aesthetic  reasons,  be  of 
reinforced  concrete  6  in.  thick,  finished,  say, 
with  roughcast.  The  roof  and  the  princi- 
pals are  of  reinforced  concrete,  the  con- 
struction being  sufficiently  shown  by  the 
cross  section,  a  feature  in  which  is  the 
arched  beam  whose  virtues  have  already 
been  briefly  noted. 


ing  covered  with  stone  will  always  be 
described  by  the  general  public  as  a  stone 
building,  of  whatever  material  the  backing 
may  consist. 

The  question,  therefore,  of  the  surface 
treatment  of  concrete  is  one  of  import- 
ance to  the  future  of  reinforced  concrete, 
especially  in  view  of  the  fact  that  the 
repeal  of  restrictive  legislation  now  per- 
mits thin  exterior  walls  (in  other  words, 
mask  walls)  to  be  employed.  The  subject 
of  surface  treatment  has  already  received 
great  attention,  but  the  matter,  is  one  for 
further  experiment ;  and  in  the  search  for 
more  pleasing  finishes — those  whose  effect 


REINFORCED    CONCRETE 


will  be  enhanced,  not  ruined,  by  the  action  By  "  natural  finishes  "  are  meant  those 

of    the    weather — the    knowledge    of    the  obtainable  simply  by  varying  the  aggregate 

chemist,  as  well  as  of  the  practical    man,  and  the  nature  of  the  facing  in  the  form,  the 

should    be   enlisted.     Definite   examples   of  surface   left    on   removing   the    false -work 

the    results    obtained,    as    well    as    of   the  undergoing  either  no  treatment   at  all   or 


Fig.  455 


Figs.  454  to  457.— 
Longitudinal  Sec- 
tion, Plan,  Front 
Elevation  and  Gross 
Section  of  Hall  with 
Reinforced  Concrete 
Principals  and  Roof 


•J* 


1 


a 


Fig.  456 

methods  ordinarily  adopted,  should  be  easily 
accessible  for  inspection  by  architects  gener- 
ally. 

The  methods  of  finishing  concrete  sur- 
faces will  now  be  considered  in  detail.  They 
may  be  classified  under  two  main  headings  : 
(1)  natural  finishes,  and  (2)  applied  finishes. 


Fig.  457 

only  sufficient  to  bring  into  relief  the  aggre- 
gate in  the  facing  or  to  make  the  texture 
uniform. 

"  Applied  finishes "  are  those  obtained 
by  the  application  of  plastic  materials. 

The  Untouched  Surface. — A  common 
finish,  but  one  that  assthetically  fails  to. 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


243 


please  many  people,  is  to  leave  the  wall  just 
as  it  comes  from  the  forms,  with  all  the 
board  marks  showing  ;  but  what  it  lacks  in 
beauty  it  makes  up  in  efficiency.  C.  K. 
Knapp,  in  a  paper  read  before  the  National 
Association  of  Cement  Users  (U.S.A.),  men- 
tioned an  instance  in  which  the  original 
cement  skin  had  been  left  undisturbed,  and 
in  which  it  defied  for  a  number  of  years  the 
penetrative  power  of  water  which  lay  in  a 
pool  over  a  ceiling.  He  stated  that  after 
protracted  storms  the  surface  seems  to  dry 
off  instantly,  while  neighbouring  houses  of 
brick,  and  even  frame  construction,  retain 
evidences  of  moisture  upon  the  outside  long 
after  the  concrete  house  has  assumed  its 
natural  colour.  Not  only,  said  Mr.  Knapp, 
was  £40  to  £60  saved  for  tool  dressing,  but, 
in  the  opinion  of  the  architect,  the  finish 
was  more  artistic,  as  well  as  indicative  of 
the  plastic  nature  of  concrete. 

It  may  be  taken  as  granted,  however,  in 
spite  of  an  occasional  opinion  to  the  con- 
trary, that  the  uninteresting  nature  of  the 
undisturbed  finish,  its  drabness,  and  its 
entire  lack  of  light  and  shade  effects,  are 
obstacles  to  the  employment  of  concrete  in 
many  quarters,  and  that  something  different 
will  nearly  always  be  demanded. 

The  Brush  Finish. — One  of  the  cheapest 
and  most  satisfactory  methods  of  treating 
the  green  concrete  is  to  give  it  a  stiff  brush- 
ing, taking  care  to  make  the  effect  as  uniform 
as  possible,  it  being  quite  easy,  either  with 
an  ordinary  scrubbing  brush  or  with  a  wire 
brush,  to  vary  the  "  pattern  "  as  the  work 
proceeds. 

It  will  be  noted  that  the  concrete  must 
be  green  ;  but  it  is  not  possible  to  give  a 
definite  time  from  the  placing  of  the  concrete 
to  the  commencement  of  the  brushing,  as 
naturally  it  will  depend  upon  the  propor- 
tions of  the  ingredients,  the  state  of  the 
weather,  etc.  Should  the  brushing  be 
attempted  before  the  concrete  is  sufficiently 
set,  the  result  will  be  irregular,  as  any  small 
aggregate  present  may  easily  be  broken 
out ;  on  the  other  hand,  should  the  brush- 
ing be  delayed  too  long,  the  concrete  may 
have  become  too  hard  to  be  treated  by 
this  method. 

The  domestic  scrubbing  brush  answers 
when  the  concrete  is  very  green,  and  the 
wire  brush  when  the  concrete  is  harder. 
It  is  customary  to  flow  water  over  the 
surface  of  the  concrete  freely  during  the 
course  of  the  brushing,  and  this  is  un- 


doubtedly most  conveniently  done  by 
means  of  a  hose  with  a  rose  head. 

It  is  customary  to  assist  the  brushing  by 
treating  the  washed  surface  with  dilute 
muriatic  or  acetic  acid,  which  helps  to  clean 
the  aggregate  and  conduces  to  uniformity 
of  texture  throughout  the  whole  job,  which 
very  possibly  has  been  scrubbed  down  at 
varying  intervals  of  time.  One  part  of  acid 
diluted  with  three  parts  of  clean  water  is 
suitable  ;  but  when  the  concrete  has  been 
made  with  white  cement  and  white  stone 
aggregate,  it  is  better  to  use  a  1  : 3  dilute 
sulphuric  acid. 

An  objection  to  the  use  of  acids  (muriatic, 
acetic,  etc.)  is  the  possibility  of  the  forma- 
tion of  stains  and  efflorescence,  which 
certainly  are  very  unsightly.  An  acid- 
treated  surface  needs  to  be  well  washed 
with  plenty  of  water. 

The  Carborundum  Finish. — The  well- 
known  carborundum  finish  gives  a  light- 
coloured  surface  and,  by  filling  in  the  pores 
with  cement,  renders  the  material  less 
pervious  to  water.  Immediately  the  forms 
are  removed,  the  surface  is  wetted  and  then 
rubbed  with  a  No.  16  carborundum  stone 
until  a  lather  forms  and  extreme  roughness 
has  been  removed.  The  work  is  washed 
down  with  the  help  of  a  brush,  and  while 
wet  dusted  with  a  1  :  2  mixture  of  cement 
and  fine  sand,  which  is  next  rubbed  in  with 
a  No.  16  stone,  the  finish  being  applied  by 
rubbing  with  a  No.  30  stone.  A  committee 
appointed  by  the  National  Association  of 
Cement  Users  (U.S.A.)  has  recommended 
this  method,  but  specified  the  use  in  the 
early  stages  of  a  No.  8  carborundum  stone 
brick  instead  of  the  (finer)  No.  16. 

The  Sand-blast  Finish.— Sand-blasting 
has  proved  a  quick  and  convenient  method 
of  finishing,  the  resultant  surface  rather 
resembling  that  produced  by  scrubbing,  but 
being  more  regular.  The  principle  is  the 
abrasion  of  the  surface  by  particles  of  sharp 
sand  carried  at  a  high  velocity  by  a  current 
of  air  under  pressure.  The  abrasive  removes 
the  board  marks  and  produces  a  uniform 
matt  finish. 

Whilst  practically  any  type  of  sand-blast 
apparatus  can  be  adapted  to  the  purpose, 
there  is  much  to  be  said  for  the  Niagara 
pattern  illustrated  by  Fig.  458.  This  is  a 
simple  T-piece  weighing  about  6  lb.,  and 
it  acts  on  a  well-known  principle  exempli- 
fied by  the  familiar  scent-sprayer.  The  air, 
under  pressure  of  from  80  lb.  to  90  lb.  per 


244 


REINFORCED    CONCRETE 


square  inch,  is  supplied  through  the  flexible 
l|-in.  hose  shown,  and,  in  popular  language, 
produces  suction  in  the  IJ-in.  vertical  pipe, 
whose  lower  end,  weighted,  is  placed  in  the 


Fig.  458. — T -piece  for  Sand-blast  Apparatus 

sand.  The  sand,  by  the  way,  can  be  held 
in  a  vessel  or  may  simply  be  in  a  heap. 
When  the  device  needs  cleaning,  the  nozzle 
is  pressed  against  a  hard  surface,  this 
causing  the  compressed  air  to  find  an  outlet 
through  the  suction  pipe  and  effectually 
clearing  this  of  any  obstruction.  Obviously, 
any  similar  material  to  sand  can  be  used  with 
the  device,  and  it  may  sometimes  be  an 
advantage  to  use  something  with  less  pro- 
nounced cutting  properties. 

Bush-hammered  Finish.  —  What  is 
known  as  "bush  hammering " — so-called 
from  the  use  of  the  mason's  bush  hammer, 
one  form  of  which  tool  is  illustrated  by 
Fig.  459 — destroys  the  facing  film  of  cement, 
and  therefore  injuriously  affects  the  water- 
tightness  of  the  work  ;  but  where  this  is  un- 
important, the  method  can  be  recommended 
as  producing  an  interesting  surface,  the 
cement  between  the  aggregate  being  chipped 
out  and  the  stone  aggregate  itself  being 
roughened.  The  work  should  be  done  within 


Fig.  459.— A  Type  of  Bush  Hammer 

about  two  months  of  setting,  as  otherwise 
the  concrete  may  be  too  hard  to  be  affected 
by  the  tool.  Stone  masons'  tools  of  the  axe 
and  patent-axe  type  will  be  familiar  to  most 
readers  ;  the  bush  hammer  is  used  in  prac- 


tically the  same  way,  either  by  hand  or  in  a 
compressed-air  machine  tool. 

Facing  Concrete  in  the  Form. — The 
full  advantage  of  most  of  the  above  methods 
of  finishing  can  only  be  realised  when  special 
precautions  have  been  taken  to  see  that  the 
work  is  given  a  facing — at  least  1  in.  thick—- 
of fine  material  in  the  form  ;  an  applied 
facing,  plastered  on  the  hard  concrete,  is 
not  the  same  thing.  The  facing  is  applied 
in  course  of  executing  the  main  concreting 
by  plastering  it  on  the  form  and  then 
introducing  the  body  concrete ;  or,  in- 
stead, the  body  concrete  is  inserted  anc 
then  pushed  back  from  the  form  to  allow  of 
the  fine  stuff  being  introduced  ;  still  another 
method  is  to  use  iron  plates,  as  described 
later.  By  "fine  stuff"  is  meant  a  mortar 
or  concrete  with  a  small  aggregate  suitable 
to  show  on  the  face  of  the  work  ;  it  certainly 
does  not  mean  excessively  rich  stuff,  as  this 
is  always  liable  to  go  "  crazy,"  that  is, 
develop  hair  cracks  all  over  its  surface. 
1  of  cement  to  from  2  to  3  of  fine  aggregate 
is  a  suitable  proportion.  When  the  facing 
is  to  contain  both  fine  and  coarse  aggregates, 
suitable  proportions  are  1  :  1J  :  3,  1:2:3, 
or  even  1:2:4,  the  ingredients  being 
cement,  sand  or  stone  screenings  and  crushed 
stone  or  screened  gravel,  and  the  facing  must 
be  twice  as  thick  as  the  diameter  of  the 
largest  stone  in  the  aggregate. 

To  facilitate  the  placing  of  both  the  facing 
mixture  and  the  backing  without  mutual 
interference,  the  use  of  iron  plates — "  grano- 
lithic plates  " — is  recommended.  These  maj 
be  of  any  suitable  depth  and  length,  12  in. 
X  5  ft.  or  6  ft.  being  suitable,  furnishec 
with  a  handle  near  each  end  and  flared  oi 
at  the  top,  as  in  Figs.  460  to  462.  On  the  side 
opposite  to  the  flare  two  or  three  angle-irons 
or  T-irons  are  riveted  to  act  as  distance 
pieces.  As  many  of  these  plates  as  requirec 
are  put  into  the  forms,  their  ends  slightlj 
overlapping,  with  the  angles  or  T's  close  to 
the  face  of  the  form,  in  this  way  forming 
a  deep,  narrow  slot  into  which  the  facing 
stuff  can  easily  be  poured,  the  flaring  of 
the  plate  assisting  this.  The  concrete  back- 
ing is  poured  in  at  the  same  time,  but  the 
facing  mixture  is  kept  at  a  slightly  higher 
level  to  prevent  any  thin  stuff  from  the 
concrete  running  over  the  top  of  the  facing 
and  finding  its  way  to  the  face  of  the  form. 
When  the  layer  is  of  the  desired  thickness, 
the  plates  are  raised  in  readiness  for  the 
next  layer,  and  the  two  materials — the  bod) 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


245 


concrete     and     the     facing — brought     into 
intimate  union  by  ramming. 

When  the  aggregate  in  the  facing  material 
has  been  selected  especially  to  give  character 
to  the  concrete,  care  must  be  taken  to  ensure 
its  fineness,  and  it  will  be  necessary  to  re- 
move the  cement  skin  from  the  face  of  the 
work.  It  is  usual  to  use  as  aggregate  stone 
or  pebbles  crushed  to  pass  through,  say,  a 
£-in.  screen,  but  to  be  retained  upon  a  |-in. 
screen.  An  average  diameter  of  f  in.  is 
about  right.  The  fine  stuff  is  applied  to  the 
face  of  the  form  by  means  of  a  trowel  or 
plasterer's  float  just  before  introducing  the 
body  concrete ;  or  the  "  granolithic  plate  " 
method  already  described  is  employed.  Care 
must  be  taken  to  get  the  two  materials 
into  intimate  union.  By  means  of  a  little 
thought  it  is  possible  to  choose  aggregates 


a  wooden  block,  or  with  sandstone  and 
plenty  of  water,  but  this  method  will  not 
leave  the  aggregate  in  relief.  Again,  bush 
hammering,  as  already  described,  may  be 
adopted.  The  following  suggestions  are  due 
to  Henry  H.  Quimby,  M.Am.Soc.C.E.  :  "  If 
the  height  of  the  wall  to  be  thus  treated  is 
too  great  to  be  completed  in  one  day,  face 
forms  must  be  constructed  to  facilitate  the 
removal  of  the  planking  without  disturbing 
the  studs  or  uprights.  This  is  easily  accom- 
plished by  setting  the  studs  8  in.  to  12  in. 
away  from  the  face  line  and  supporting 
planks  with  cleats — say  2-in.  by  1-in. — 
tacked  to  the  studs  and  the  planks.  This 
permits  the  lower  planks  to  be  removed  and 
the  washing  done  while  the  upper  planks 
are  in  place  and  concrete  is  being  deposited. 
With  the  exercise  of  very  watchful  care  on 


fit.  460 


Fig.  461 


[    C 


Fig.  462 
Figs.  460  to  462.— Front  and  End  Elevations  and  Plan  of  "Granolithic  Plate" 


of  different  colours  and  sizes  to  produce 
excellent  effects,  and  a  note  may  here  be 
made  of  the  suitability  of  selected  ground 
mica  for  this  purpose,  this  material  supply- 
ing "  life  "  and  sparkle  suggesting  freshly 
wrought  granite.  The  next  part  of  the  pro- 
cedure will  depend  largely  upon  the  nature 
of  the  work  and  the  setting  power  of  the 
cement  used.  All  that  can  here  be  said  is 
that  the  surface  of  the  granolithic  con- 
crete must  be  exposed  while  the  material 
is  still  friable,  so  that  an  immediate  wash- 
ing with  water  and  a  stiff  brush  will  remove 
the  cement  film  and  expose  the  aggregate, 
which  will  now  appear  in  decided  relief 
and  of  a  rough,  coarse  texture.  (The  illus- 
trations, Figs.  463  to  468,  on  later  pages 
are  full-size  photographic  reproductions  of 
patterns  obtained  by  the  above  method, 
and  the  inscriptions  give  all  necessary 
particulars.  They  are  due  originally  to 
Engineering  Neivs")  Even  when  the  face  has 
become  too  hard  for  brushing,  something  of 
the  effect  can  be  produced  by  rubbing  with 


the  part  of  the  workmen  and  unremitting 
inspection,  two  different  days'  work  can  be 
joined  so  that,  after  washing,  the  joint  will 
not  be  unsightly — even  scarcely  distinguish- 
able ;  but  such  work  is  usually  not  obtain- 
able throughout  a  structure,  and  it  is  found 
very  easy  to  obtain  thoroughly  satisfactory 
joints  by  indenting  horizontal  grooves  at 
regular  intervals  representing  courses,  and 
finishing  each  day's  work  at  the  apex  of 
a  groove.  These  indentations  are  made  by 
means  of  triangular  beads  on  the  face  forms. 
Usually  the  bead  is  the  bevelled  edge  of  a 
strip  set  between  the  face  planks  and  lightly 
secured  to  the  planks  with  partly  driven 
toe  nails,  so  that,  if  desired,  a  plank  can  be 
removed  independently  of  the  bead  above 
it,  the  bead  remaining  to  set  the  plank  upon 
the  next  course.  These  grooves  in  the  face 
of  a  wall  improve  the  appearance  by  re- 
lieving the  blankness  of  a  large  area.  It  is 
found  practicable  to  prosecute  the  work 
with  one  course  of  planks  where  the  capacity 
of  the  plant  for  one  day  is  equal  to  only  one 


246 


REINFORCED    CONCRETE 


course  of  concrete.  In  this  way  the  same 
planks  have  been  used  for  many  different 
courses  on  four  or  more  different  structures." 


Fig.  463.— Sand  Concrete,   1  :  2.     Full  size 


Uniformity  of  aggregate, 
both  as  regards  size  and 
colour,  is  of  great  import- 
ance in  this  method  of 
finishing.  The  matter  needs 
careful  attention  before 
any  work  is  started  in 
order  to  minimise  the  risk 
of  being  obliged  to  com- 
plete the  walling  with 
aggregate  of  a  different 
character. 

The  nature  of  the  tamp- 
ing exercises  a  great  influ- 
ence on  the  texture  of  the 
face  of  the  work.  For 
example,  when  a  large 
aggregate  is  wished  to 
show  in  the  face  of  a  wall, 
the  tamping  should  be  done 
in  the  middle,  this  carry- 
ing the  stones  against  the 
form.  Tamping  near  the 
side  helps  in  getting  the 
finer  stuff  towards  the  face. 
In  producing  horizontal, 
flat  surface,0,,  skilful  tamp- 


ing will  bring  the  finer  stuff  to  the  top, 
but  a  special  tamper — the  "  Andrews,"  of 
American  origin,  but  obtainable  in  both  the 
United  States  and  Great 
Britain — has  been  intro- 
duced to  facilitate  this 
particular  work.  As  shown 
on  p.  129,  the  tamper 
has  a  number  of  pyramidal 
points  on  its  working  sur- 
face, so  arranged  that 
when  a  mass  of  concrete 
containing  large  particles 
of  stone  and  gravel  is 
tamped,  these  larger  par- 
ticles are  pushed  down 
further  into  the  mass,  and 
the  thin  stuff  flows  to  the 
surface.  The  tool  is  8  in. 
square,  and  the  points 
projecting  from  the  face 
are  connected  at  their 
bases  in  such  a  way  as  to 
prevent  particles  of  stone 
from  becoming  jammed 
between  them.  In  use, 
the  tamper  is  employed 
after  the  layer  of  concrete 
has  been  spread  and 
roughly  levelled.  After  its 


Fig.   464. — Crushed  Stone  Concrete  (Cement   1,   Yellow  Bank 
Sand  2,   and  f  in.   Screened  Stone  3).     Full  size 


ARCHITECTURAL   AND    SURFACE   TREATMENT 


247 


use,  the  surface  may  be  smoothed   off  and 
finished  like  any  other  finish  dressing. 
To  cause  the  thin  stuff  in  the  wet  mixture 


Fig.   465. — Pebble  Concrete  with  Scrubbed  Surface  (Cement   1, 
Bar  Sand  2,  and  ^  in.   White  Pebbles  3).     Full  size 


to  flow  to  the  side  of  the 
form,  and  thus  obtain  a 
fine  vertical  facing  with- 
out introducing  a  second 
mixture,  the  use  of  a  per- 
forated spade  (see  p.  128) 
has  been  suggested ;  and 
many  contractors  will  find 
it  convenient  to  carry  out 
experiments  with  the  ob- 
ject of  applying  the  idea 
in  other  ways.  A  spade 
illustrated  on  p.  129  is 
that  known  as  the  Ross. 
Its  action  is  to  force  back 
the  coarser  aggregate  and 
to  allow  the  finer  stuff  to 
come  to  the  face  of  the 
work.  Other  tools  for  a 
similar  purpose  are  illus- 
trated on  pp.  128  and  129. 
Pebble  Dashing.  — To 
imitate  the  effect  of  stucco 
upon  which  pebbles  have 
been  dashed,  the  face  of 
the  form  is  plastered  with 


well-worked  wet  clay  into  which  have  been 
lightly  pressed  small  pebbles,  pieces  of  glass 
or  glazed  tile,   marble   chips,   etc.     When, 
after  the  concrete  has  set, 
the  form  is   taken   away, 
the  clay  is  washed  off  and 
the  surface  brushed  to  re- 
veal  the  pebbles,  etc.,  in 
the  face  of  the  concrete. 

Sand  Finish. — The  me- 
thod is  on  the  lines  of  that 
described  in  the  preceding 
paragraph.  Clay  is  well 
tempered  and  thoroughly 
worked,  and  then  plas- 
tered on  the  inside  of  the 
forms,  working  it,  if  de- 
sired, into  patterns.  Sand 
is  applied  evenly  to  the 
wet  clay,  and  then  the 
concrete  is  poured  in.  On 
the  removal  of  the  forms, 
the  clay  is  washed  off  with 
brush  and  water,  the  sand 
adhering  to  the  concrete. 
This  method  certainly  ap- 
pears to  be  unnecessarily 
troublesome.  Merely  to 
get  a  sand  finish  on  plain, 
flat  walling,  it  should 
suffice  to  .brush  over  the 


'    •*'•  KLI  ~ 

mm 


f&  .'>•    '•  Y*»  1 

A'I'O  -r£ 

Af\*4&;.>       &r$«ai 


Nt^-C?^, 

s»o? 


&•-• 


•^•••M 

Fig.   466. — Granite  Grit  Concrete  (Cement   1,   Bar  Sand  2,   and 
i  in.  Granite  Grit  3).     Full  size 


REINFORCED    CONCRETE 


form  with  clay  water,  and  apply  the  sand, 
which  will  readily  adhere. 

Glazed  Finish. — Whilst  concrete  cannot 
be  truly  polished,  it  is  not  difficult  in  theory 
to  obtain  on  it  a  glazed  surface.  All  that 
would  be  necessary  would  be  to  line  the 
form  where  required  with  well-cleaned  and 
polished  glass,  or,  as  an  American  experi- 
menter has  proposed,  with  enamelled  iron 
plates,  of  the  kind  universally  employed  for 
large  advertising  signs.  In  practice,  the 
use  of  such  an  untractable  and  brittle 
material  as  glass  would  cause  never-ending 
difficulty,  although  it  might  be  possible 
without  much  inconvenience  to  introduce 
narrow  slips  for  the  purpose  of  casting 
ornamental  devices  on  the  work.  The 
employment  of  steel  forms  stove-enamelled 
on  the  working  side  might  provide  a  solution 
to  the  problem  should  the  production  of 
glazed  surfaces  be  demanded.  A  practical 
difficulty  would  be  the  almost  inevitable 
formation  on  the  form  of  air-bells,  as  these 
would,  of  course,  spoil  the  surface  ;  but  the 
trouble  might  be  got  over  by  first  lining  the 
form  with  fine  cement  mortar  \  in.  or  less 
in  thickness,  applying  it  with  heavy  pres- 
sure. It  is  reasonable  to  assume  that  the 
polished  skin  would  give  the  concrete  more 
than  usual  resistance  to  weather  and  to 
mechanical  wear. 

APPLIED   FINISHES 

Plastered  Surfaces. — When  a  concrete 
surface  is  finished  by  coating  with  plaster 
of  whatever  nature,  there  is  always  a  risk 
of  this  coming  away  at  a  later  date  should 
there  be  a  likelihood  of  moisture  percolating 
through  the  concrete  and  affecting  the  back 
of  the  plaster.  However,  plastering  is  often 
successful,  but  much  depends  upon  the  pre- 
paration of  the  concrete  surface  and  the 
kind  of  plaster  used.  A  good  plan  is  to 
wash  the  concrete  with  dilute  acid,  talcing 
care  to  rinse  this  off  afterwards,  and  then, 
while  the  surface  is  damp,  but  not  flooded 
with  water,  to  apply  the  plaster.  The 
moisture  in  the  surface  of  the  concrete 
assists  the  cement  mortar  to  set.  The 
following  method,  advanced  by  an  American 
contractor,  should  give  good  results.  The 
first  coat  is  a  1  :  3  mixture  as  dry  as  it  can 
be  applied,  this  necessitating  considerable 
pressure  to  cause  it  to  adhere  well.  Let 
the  coat  be  thin  and  scratch  it  afterwards. 
Brush  over  the  first  coat  with  water  and 
apply  the  second  coat,  using  as  much  pres- 


sure as  possible.     If  a  third  coat  is  neces- 
sary, adopt  the  same  precautions. 

A  concrete  surface  gives  a  better  key  for 
plaster  after  it  has  been  hacked  over  with 
axe  or  bush  hammer,  but  this  necessitates  a 
thick  coat  of  plaster.  If  the  concrete  is  soft 
enough  to  be  affected  by  a  coarse  wire 
brush,  a  rub  down  with  that  will  answer 
instead  of  hacking,  and  the  plaster  coat 
can  then  be  thin.  The  acid  treatment  and 
washing,  as  described  on  p.  243,  should 
precede  the  plastering. 

Tiles,  Mosaic,  Sgraffito,  etc. — In  some 
situations,  tile,  or  the  old  Byzantine  material, 
vitrified  mosaic,  makes  a  beautiful  and  suit- 
able finish.  In  either  case,  they  would  have 
one  constructional  advantage,  especially  in 
high  buildings,  over  brick  or  masonry 
casings,  in  that  the  weight  added  by  them 
to  the  foundations  would  be  comparatively 
trifling.  A  concrete  wall  having  a  true,  flat 
surface  could  be  prepared  for  tiling  by 
rubbing  down  with  a  carborundum  stone  and 
then  scratching  with  a  brush — bristle  or  wire, 
according  to  the  age  of  the  concrete  ;  the 
wall  should  be  wetted  and  the  tiles  bedded 
in  neat  cement,  using  as  small  a  quantity 
as  possible  to  get  a  good  result.  Much  the 
same  instruction  applies  to  the  mosaic. 

It  is  somewhat  surprising  that  sgraffito  is 
not  more  generally  adopted  as  a  finish  to 
concrete  work  where  ornamental  effects  are 
desired  ;  certainly  in  some  cases  it  might 
be  very  effectively  employed.  Sgraffito  is 
an  Italian  word  meaning  "scratched,"  and 
is  the  name  of  a  very  old  process  consisting 
in  applying  to  the  surface  several  coats  of 
different  colours,  these  coats  being  brought 
to  view  as  required  by  chipping  or  scraping. 
In  the  simplest  form  there  are  but  two 
coats — black  for  the  ground,  white  for  the 
covering  ;  and  the  surface  to  be  treated  is 
floated  to  a  uniform  face,  allowing  for  the 
finishing  coat,  which  will  be  £  in.  thick. 
The  outline  of  the  design  is  marked  on  the 
face  of  the  floating,  as  a  guide  for  keying 
the  J-in.  finishing  coat,  which  should  be 
applied  as  soon  as  the  colour  coat  is  suffici- 
ently hard.  Placing  the  design  drawn  on 
paper  in  its  original  position,  pounce  through 
the  outline  as  a  guide  for  cutting  through 
to  the  colour  coat,  using  a  worn  knife  to 
cut  away  the  superfluous  material,  which  is 
removed  with  a  spatula  ;  the  edges  of  the 
work  being  sloped  or  inclined  according  to 
the  light  or  shade  required.  For  three-coat 
work  the  colour  coat  may  be  left  rough, 


ARCHITECTURAL    AND    SURFACE    TREATMENT 


249 


providing  it  is  uniform,  a  dark  colour  being 
used  as  a  background.  This  coat  is  ruled 
to  within  f  in.  of  the  finished  face,  the  class 


Wjf£*    •      . 


Screened  Yellow  Pebbles  3).     pull 


tained  by  adding  a  sample  of  it  to  a  small 
quantity  of  the   stuff  with  which  it  is  to 
be    used,  the   material    being   allowed   full 
time  for  setting.     For  ex- 
^•HH^HI      ternal  work,  portland   ce- 
ment   or    Aberthaw    lime 
may  be  used,  in  the  pro- 
portion of  2  parts  sand  to 
1  part  cement.     Aberthaw 
lime  may  also  be  used  for 
internal  work,  and  so  also 
may  Parian  cement ;    but 
for  ordinary  purposes    se- 
lenitic may  be  used  with 
satisfactory  results. 

Fig.  469  (p.  251)  shows 
the  excellent  effect  ob- 
tained by  the  adoption  of 
inlaid  faience  tiles  and  red 
bricks,  the  roof  covering 
consisting  of  terra-cotta 
tiles. 

"Stuc"  Work.— This 
is  a  form  of  interior  treat- 
ment particularly  suitable 
for  application  to  concrete 
walls  and  lending  itself  to 
a  simple  and  effective  de- 
sign. In  appearance,  it 
suggests  a  good  Bath  stone. 
The  material  is  made  with 


Sand  2,  and 
size 


of  work  determining  the. 
thickness  of  the  various 
coats.  Apply  the  succeed- 
ing coat,  and  finish  as 
soon  as  convenient,  so  that 
the  coats  may  adhere  in 
one  compact  mass.  Expe- 
dition is  specially  neces- 
sary when  treating  exterior 
work,  as  water  settling  on 
the  incisions  of  work  im- 
properly keyed  would  cause 
it  to  laminate  or  scale  off. 
For  colouring  matter,  to 
obtain  good  black  use 
bone-black — or,  for  ordin- 
ary work,  smiths'  ashes 
as  an  aggregate ;  for  red, 
Venetian  or  Indian  red ; 
for  brown,  umber ;  for 
yellow,  yellow  ochre.  When 
a  neutral  tint  is  required, 
a  combination  of  two  or 
more  colours  may  be  em- 
ployed. The  depth  of  the 
colour  should  be  ascer- 


Fig.  468.— Sand  Concrete,   1  :  3 


250 


REINFORCED    CONCRETE 


a  mixture  of  plaster-of-paris,  cement,  size  and 
various  colouring  matters,  the  latter  vary- 
ing slightly  according  to  the  desired  finished 
effect.  In  the  application,  a  very  moist  solu- 
tion of  plaster-of-paris  is  first  stippled  on  the 
walls  with  a  stiff  broom  to  give  a  key  for  the 
"  stuc."  The  plaster  and  cement  are  then 
placed  in  a  mixing  box  about  2  ft.  by  1  ft.  5  in. 
and  about  12  in.  deep,  and  thoroughly 
mixed.  When  this  is  complete,  the  size, 
water  and  colours  are  added,  and  the  whole 
thoroughly  incorporated,  and  the  mass  is 
squeezed  through  the  hands  until  a  thick 
cream  is  obtained.  The  resulting  compound 
is  very  quick  in  setting,  and  consequently 
it  requires  to  be  rapidly  applied  to  the  sur- 
faces, these  having  been  previously  screeded. 
The  hands  and  a  brass  trowel  are  used,  the 
plaster  being  literally  thrown  on.  Whilst 
moist,  it  is  roughly  lined  to  the  screeds,  and 
the  surfaces  are  planed  and  scraped  to  a 
fairly  even  and  true  face.  When  set,  it  is 
left  for  about  two  months,  and  during  this 
period  various  stains  come  out  to  the  face 
of  the  "  stuc,"  which  assumes  a  light  brown 
colour,  and  becomes  very  hard.  After  this 
time  a  scraping  plane  is  employed  and  the 
whole  of  the  outer  surface  is  removed,  taking 
away  all  stains  and  leaving  a  fine  soft  stone 
effect.  During  the  scraping  process,  small 
pit  holes  are  formed  in  the  surface,  which 
greatly  improves  the  appearance,  and  re- 
moves the  perfectly  smooth  and  artificial 
effect  otherwise  given.  The  whole  surface 
is  then  rubbed  down  with  pumicestone,  and 
the  various  joints  are  set  out  with  chalked 
cords  in  accordance  with  the  design.  These 
lines  are  then  cut  in  and  filled  up  with  white 
plaster,  and  a  final  rubbing  down  with 
pumicestone  is  given.  Mouldings  and  cor- 
nices are  run  in  the  orthodox  manner  with 
a  "  horse,"  several  plasterers  throwing  on 
the  material  at  each  point,  whilst  the  horse 
is  worked  backwards  and  forwards. 

When  using  the  "  stuc  "  on  concrete  work 
it  is  advisable  to  give  the  walls  a  thin  coat- 
ing of  Keene's  cement  to  prevent  the  staining 
of  the  "  stuc." 

In  the  execution  of  the  ornament,  some 
excellent  work  is  seen,  as  this  is  not  cast, 
but  modelled  in  position  by  the  plasterer, 
and  for  this  reason  every  man  requires  to  be 
an  artist  fully  in  sympathy  with  his  work. 
"  Stuc  "  work  was  extensively  adopted  in  the 
decoration  of  the  Koyal  Automobile  Club, 
and  it  can  be  said  confidently  that  the  work 
to  the  fluted  columns  round  the  gallery  and 


the  Eoman-Doric  caps,  with  the  enriched 
frieze  above,  are  greatly  superior  to  similar 
work  executed  in  ordinary  fibrous  plaster. 
The  "  stuc  "  work  requires  neither  painting 
nor  other  finish,  and  retains  its  fine  appear- 
ance if  merely  rubbed  down  once  every  three 
years. 

COLOURING    CONCRETE 

In  response  to  a  demand  for  a  brighter  and 
more  interesting  colour  than  that  provided 
by  natural  concrete,  there  has  been  a  great 
deal  of  experiment  to  determine  the  best 
methods  by  which  the  desired  result  can  be 
obtained.  Undoubtedly,  the  most  artistic 
method,  but  one  not  suited  to  all  or  even 
the  majority  of  situations,  is  to  determine 
the  colour  by  the  use  of  a  specially  selected 
aggregate  which  will  give,  not  only  colour, 
but  texture  to  the  work.  Sufficient  has  been 
said  on  this  subject  earlier  in  this  chapter. 
Another  method  is  to  face  the  walls  with 
tiles,  mosaic,  etc.,  as  already  described,  but 
this  is  expensive,  and  has  the  disadvantage 
of  masking  the  character  of  the  concrete 
surface,  and  in  that  particular  sense  is  false 
art.  Strictly,  methods  of  colouring  concrete 
include  only  those  by  which  (a)  pigment  is 
incorporated  with  the  concrete  or  with  the 
facing  material  introduced  into  the  form, 
(6)  staining  or  (c)  distempering,  or  (d)  oil- 
painting  the  concrete  surface. 

Body  Colours. — But  for  the  fact  that  the 
lime  in  the  concrete  has  an  injurious  effect 
upon  the  majority  of  pigments,  there  would 
be  very  little  to  say  under  this  heading. 
This  injurious  action  has  long  been  known, 
and  everyone  is  familiar  with  the  bleaching 
of  paint  and  wallpaper  by  the  lime  in  newly- 
plastered  walls.  The  vegetable  and  many 
of  the  artificial  pigments  are  the  worst 
offenders,  leaving  the  selection  between  the 
relatively  few  mineral  pigments.  A  com- 
mittee appointed  by  the  National  Association 
of  Cement  Users  (U.S.A.)  has  reported  that 
the  only  mineral  pigments  that  should  be 
used  are  comprised  in  the  following  list : 
Lampblack,  manganese  dioxide,  red  iron 
oxide,  English  red  oxide,  brown  roasted  iron 
oxide,  brown  ochre,  yellow  ochre,  ultra- 
marine, chromium  oxide,  ultramarine  green 
and  violet  oxide  of  iron.  This  brief  list  of 
pigments  does  not  allow  of  much  latitude. 
The  committee  further  reports  that  the 
amount  that  can  be  safely  used  is  small 
owing  to  the  danger  of  impairing  the  strength 
of  the  concrete.  Five  per  cent,  by  weight 


251 


252 


REINFORCED    CONCRETE 


to  that  of  the  cement  should  be  the  limit, 
beyond  which  the  impairment  of  strength 
is  too  great  to  justify  a  larger  amount, 
except  in  special  circumstances,  as,  for 
example,  in  a  thin  facing.  Even  this  pro- 
portion of  those  pigments  whose  colour 
differs  little  from  that  of  the  cement  does 
not  produce  a  marked  change.  Lampblack, 
it  is  pointed  out,  is  the  principal  pigment 
used  for  darkening,  on  account  of  its  strong 
contrast  and  the  fineness  of  its  particles. 
For  lightening  the  colour  somewhat  lime  is 
the  best  material  to  use.  Colouring  matters 
can  be  used  in  either  a  dry  form  or  as  a 
paste.  It  will  generally  be  found  most 
convenient  for  mixing  to  use  the  dry  form, 
thoroughly  mixing  it  with  dry  mortar  before 
the  addition  of  any  water. 

The  pigments  exercise  an  influence  on 
the  setting  properties.  Crimson  lake  (alum- 
ina base)  and  barium  chromate  quicken  the 
cement ;  manganese  dioxide,  red  ochre,  and 
chinese  red  retard  it ;  and  ferric  oxide, 
yellow  ochre,  ultramarine,  and  chromium 
oxide  have  a  slight  quickening  effect. 

In  colouring  concrete  by  the  admixture  of 
pigment,  the  resultant  tint  must  not  be 
judged  by  that  of  the  wet  mixture,  the  con- 
crete always  drying  out  lighter  than  that 
would  suggest.  The  best  course  is  to  mix 
together  small  measured  quantities  of  all 
the  ingredients  dry,  add  the  water,  thor- 
oughly incorporate,  and  allow  a  portion  to 
set  in  a  mould  ;  the  experiment  can  be 
repeated  until  the  desired  result  is  obtained. 

For  a  white  concrete,  use  1  of  white  port- 
land  cement  and  2  of  hard  marble,  screening 
to  pass  through  a  No.  8  screen  and  be  col- 
lected on  a  No.  40  screen.  When  the  con- 
crete is  hard,  in  three  or  four  days,  it  may 
be  rubbed  with  a  terrazzo  stone  to  polish 
the  marble  particles. 

Before  giving  a  table  of  concrete  colouring 
pigments,  it  is  desirable  to  summarise  the 
experiments  carried  out  in  the  United  States 
by  Prof.  Charles  E.  Pellew  with  the  object 
of  finding  the  most  suitable  pigments  for 
colouring  concrete.  The  question  of  expense 
was  of  vital  importance,  and  in  the  matter 
of  first  cost  it  was  evident  that  surface 
colouring  would  be  cheaper.  But,  unless 
there  is  a  thoroughly  hard  permanent  sur- 
face for  the  pigments  to  adhere  to,  and 
to  obviate  as  far  as  possible  the  use  of 
linseed  oil,  upon  which  the  lime  exerts  a 
strong  action,  the  body  colouring  is  pro- 
bably the  most  satisfactory.  For  yellow, 


Prof.  Pellew  thinks  that  the  only  available 
pigment  is  yellow  ochre,  8  per  cent,  of  a 
strong  pigment  giving  a  bright  tan  colour. 
This  yellow  can  be  used  for  shading  the  red, 
but  is  not  so  effective  for  this  as  the  man- 
ganese brown.  Yellow  ochre  with  small 
amounts  of  permanganate  brown  will  give 
various  shades  of  yellowish  brown  or  buff 
colour.  BlacJc  is  obtained  by  using  carbon 
black  or  lampblack.  For  a  bluish  shade  a 
black  iron  oxide,  imported  for  the  use  of 
gas  works,  gave  satisfaction.  The  only  red 
pigments  available  are  red  oxides  of  iron, 
some  of  them  natural,  finely  ground  haema- 
tites, and  others  artificial.  They  differ 
greatly  in  shade,  price,  and  staining  power, 
the  cheaper  pigments  being  unsatisfactory. 
It  is  best  to  use  a  small  amount  of  a  strong 
though  high-priced  pigment  than  larger 
quantities  of  a  weak  but  cheap  one.  After 
experimenting  with  twenty  or  more  different 
colours  from  various  manufacturers,  the 
best  results  were  obtained  from  a  red  colour 
at  five  cents  (2|d.)  a  pound  ;  from  7J  to 
10  per  cent,  (of  the  weight  of  cement)  was 
needed  to  give  a  full  shade.  A  slight  addi- 
tion of  permanganate  brown  gave  a  red 
terra-cotta  shade.  As  regards  brown,  a 
vegetable  pigment  proved  unsatisfactory. 
Prof.  Pellew  tried  iron-rust,  the  action  of 
which  is  based  on  the  formation  in  the 
concrete  of  a  reddish-brown  deposit  of  ferric 
hydroxide  by  the  action  of  the  lime  of  the 
cement  on  a  soluble  salt  of  iron,  like  ferric 
chloride  or  ferric  sulphate.  Unfortunately, 
it  takes  25  to  30  per  cent,  of  ferric  sulphate 
(of  the  weight  of  concrete)  to  get  at  all  a 
decided  colour  with  this  compound,  and  this 
is  a  serious  drawback.  He  tried  manganese 
brown,  which  is  based  on  the  formation  in 
the  concrete  of  brown  manganese  hydroxide 
by  the  reduction  of  the  salt  potassium  per- 
manganate. The  latter  possesses  a  strong 
rich  purple  colour,  which  in  the  presence  of 
oxi disable  material,  such  as  organic  matter, 
turns  at  once  to  a  full  seal  brown.  In  stain- 
ing concrete  the  organic  matter  must  be 
supplied  in  the  form  of  glucose  or  sugar, 
which  in  quite  small  quantities  will  change 
the  deep  purple  colour  of  the  permanganate 
into  a  rich  seal  brown.  To  get  a  deeper 
colour,  Prof.  Pellew  was  obliged  to  use 
24|  per  cent,  (by  weight  of  the  cement)  of 
permanganate  and  about  \  per  cent,  of 
glucose.  With  regard  to  a  green  colour  the 
high  price  of  chromium  oxide,  the  only  green 
mineral  pigment  that  will  stand  the  action 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


253 


of  lime,  prevents  its  general  use.  In  the 
absence  of  a  strong  blue  that  will  stand  the 
action  of  lime,  it  is  not  possible  to  obtain 
a  good  green  by  modifying  the  colour  pro- 
duced by  yellow  ochre.  Ultramarine  blue 
is  fast  to  lime,  but  its  staining  power  is  low 
when  mixed  with  other  pigments,  while 
Prussian  blue  is  easily  attacked  by  the 
cement,  and  is  not,  therefore,  included  in 
the  table  of  pigments  given  on  this  page. 
It  seems  probable  that  for  greens  some 
form  of  surface  colouring  will  have  to 
answer,  but  much  remains  to  be  done  in 
the  way  of  experiment. 

TABLE   OF    PIGMENTS 

The  weights  of  pigments  given  in  the 
following  table  are  those  recommended  for 
adding  to  each  cubic  foot,  reckoned  as  90  lb., 
of  cement ;  but  the  table  must  not  be  taken 
quite  literally,  as  experiments  have  obtained 
widely  different  results.  No  greater  claim 
can  be  made  for  the  table  than  that  it  will 
act  as  a  rough  guide.  It  cannot  do  more, 
owing  to  the  great  differences  frequently 
existing  between  two  pigments  passing  under 
the  same  name.  They  may  be  unlike  one 
another  chemically,  physically  and  in  their 
percentage  content  of  inert  cheapener  which 
so  seriously  affects  the  staining  power.  The 
pigments  mentioned  will  certainly  give  the 
tints,  but  the  proportions  must  be  adapted 
as  required,  depending,  as  above  suggested, 
on  the  strength  of  the  pigments  them- 
selves and  also  on  the  proportions  of  the 
concrete. 

BLACK  .  .  10  lb.  of  manganese  di- 
oxide or  lampblack. 

— ,  BLUISH       .  8  lb.  or  9  lb.  of  black  iron 
oxide. 

BLUE  .  .  .  3£  lb.  to  4|  lb.  of  ultra- 
marine. 

,  VIOLET       .  4J  lb.  of  violet  oxide  of 

iron. 

BROWN        .         .  4  lb.  of  brown  ochre. 

,  CHOCOLATE  6  lb.  of  manganese  di- 
oxide, 4  lb.  of  red  oxide 
of  iron,  and  2  lb.  of 
black  oxide  of  iron  or 
copper  ;  or  3|  lb.  of 
burnt  umber. 

BUFF  .  .  .  3J  lb.  of  yellow  ochre  ;  or 
4J  lb.  of  yellow  ochre 
and  2£  lb.  of  perman- 
ganate brown. 

GREEN  .  .  10  lb.  of  chromium  oxide 
or  ultramarine  green. 


GREY 

PINK  . 
— ,  DULL 


-,  BRIGHT 
-,  DULL 


SLATE 


-,  BLU»E 
-,  PINKISH 


TAN    . 

TERRA-COTTA 


YELLOW 
WHITE 


1  lb.    to    3J     lb.      of 
black     or     manganese 
dioxide. 

3  lb.  of  crimson  lake 
(alumina  base). 

2  lb.  to  3£  lb.  of  Venetian 
red. 

|  lb.  of  burnt  umber. 

2  lb.  to  3J  lb.  of  Chatta- 
nooga iron  ore  or  red 
iron  ore. 

5  lb.  of  Pompeiian  or 
English  red. 

5  lb.  raw  iron  dioxide  ; 
carbon  black  may  be 
added. 

|  lb.  to  3£  lb.  of  lamp- 
black or  manganese 
dioxide. 

1  lb.  to  31  lb.  of  ultra- 
marine blue. 

|  lb.  to  1  lb.  of  Venetian 
red  ;  or  £  lb.  of  Chatta- 
nooga iron  ore  or  red 
iron  ore. 

7  lb.  of  yellow  ochre. 

English  red  with  per- 
manganate brown  ;  or 
2  lb.  to  31  lb.  of  Chat- 
tanooga iron  ore  or  red 
iron  ore. 

5  lb.  to  10  lb.  of  yellow 
ochre. 

44 Jib.  of  barytes  (barium 
sulphate). 


Stains. — Manufacturers  have  placed  upon 
the  market  a  number  of  stains  suitable  for 
use  on  concrete.  A  brown  stain  is  easily 
made  by  dissolving  iron  sulphate  (green 
copperas)  in  water,  the  proportion  being 
about  2^  lb.  per  gal.  The  colour  is  strength- 
ened by  repeating  the  application.  The 
addition  of  alum  to  the  above  solution  gives 
a  pale  yellow  ;  whilst  chrome  alum  gives  a 
green.  Stains  have  the  advantage  over 
paints  that  they  do  not  cover  the  concrete 
with  a  thick  coating,  but  sink  into  the  pores 
and  preserve  the  natural  texture,  while  at 
the  same  time  colouring  it  in  rich,  deep  tones. 
They  can  be  applied  easily  and  rapidly,  their 
covering  power  is  two  or  three  times  that 
of  oil  paint,  and  they  cost,  bulk  for  bulk, 
much  less. 

Distempering. — Concrete  can  be  suc- 
cessfully distempered  with  common  white- 
wash or  with  cement  washes ;  as  regards 


254 


REINFORCED    CONCRETE 


colours,  the  pigments  used  must  be  fast  to 
lime  (see  under  the  heading  "  Body 
Colours").  It  will  be  better  to  kill  the 
alkali  in  the  concrete  surface  with  dilute 
acid  and  then  well  wash  with  water  before 
applying  the  distemper. 
'  Painting. — By  this  term,  of  course,  "  oil 
painting  "  is  understood.  Painting  is  desir- 
able only  on  smooth  concrete,  the  con- 
structional nature  of  which  it  is  not  desired 
to  emphasise.  The  paint  forms  a  film  of 
metallic  oxides,  sulphates,  etc.,  and  oxidised 
linseed  oil  on  the  face  of  the  work,  closing 
the  pores  and  undoubtedly  adding  to  the 
weather-resisting  properties.  Unfortunately, 
the  lime  exerts  a  violent  chemical  action 
upon  linseed  oil,  and  special  preparation  of 
the  concrete  surface  is  absolutely  essential. 
Usually,  this  preparation  consists  in  killing 
the  alkali  and  then  coating  with  at  least 
two  coats  of  good  white-lead  paint  before 
applying  the  finishing  coat.  But  a  different 
style  of  treatment  was  suggested  some  time 
ago  before  the  American  Society  for  Testing 
Materials  by  Charles  Macnichol,  who  spoke 
from  the  experience  of  many  years.  He 
primes  the  concrete  with  zinc  sulphate 
ground  up  with  an  equal  quantity  of  water. 
Then,  after  two  or  three  days,  the  concrete 
can  be  painted  on  safely.  Dr.  A.  S.  Gush- 
man  suggests  the  explanation  that  a  chemical 
reaction  results  in  the  formation  of  gypsum 
(calcium  sulphate)  and  zinc  hydroxide 
(hydrated  oxide  of  zinc),  which  substances, 
held  within  the  pores  of  the  cement,  do  not 
affect  linseed  oil  injuriously — indeed,  they 
are  common  paint  pigments.  The  method 
is  economical  as  regards  the  after  coats, 
because  the  suction  of  the  cement  has  been 
already  satisfied. 

The  architect  will  usually  hesitate  to 
specify  any  particular  formula  to  be  fol- 
lowed in  painting  concrete,  and  will  generally 
content  himself  by  specifying  one  of  the 
several  compositions  on  the  market  that  are 
specially  made  for  the  purpose,  the  con- 
stituents of  which  are,  for  obvious  reasons, 
kept  secret.  Some  of  them  are  the  result 
of  very  careful  trials,  and  Dr.  Macmimilian 
Toch,  in  a  lecture  delivered  some  time  ago 
before  the  Paint  and  Varnish  Society, 
hinted  that  the  best  results  might  be  ob- 
tained by  a  proper  admixture  of  Menhaden 
fish  oil  and  tung  or  Chinese  wood  oil.  What- 
ever paint  or  preparation  is  used,  it  is 
generally  recognised  that  it  is  useless  to 
apply  it  to  green  or  fresh  concrete,  but  an 


exposure  of  at  least  a  year,  or  even  two  years, 
will  render  the  surface  fit  for  receiving  the 
paint.  In  actual  practice,  however,  it  is 
usually  desired  to  finish  the  building  almost 
immediately  after  it  is  erected,  and  in  such 
a  case  it  becomes  necessary  to  prepare  the 
surface  in  order  that  the  paint  may  per- 
manently adhere.  Even  when  the  concrete 
has  been  exposed  for  a  year  or  more  the 
application  of  such  a  preparation  may  be 
made  as  a  matter  of  precaution  to  neutralise 
any  free  lime  which  might  remain. 

Opinion  widely  differs  as  to  the  com- 
position of  this  preparation.  Many  practical 
men  advocate  the  use  of  a  solution  of  zinc 
sulphate,  commonly  known  as  "  white 
vitriol."  This  is  used  in  the  proportion  of 
8  Ib.  to  1  gallon  of  water.  It  is  urged  against 
this  method  that  something  more  than  a 
neutralisation  of  the  free  lime  is  required, 
and  that  to  obtain  satisfactory  results  a 
material  should  be  used  which  will  fill  up 
the  pores  of  the  concrete  and  prevent 
suction.  Hence  there  are  advocates  of 
compositions  in  which  paraffin  wax  plays 
an  important  part,  a  method,  it  may  be 
remarked  in  passing,  which  was  successfully 
employed  a  year  or  so  back  in  connection 
with  Cleopatra's  Needle  on  the  Thames 
Embankment. 

Under  ordinary  conditions,  however,  the 
most  practical  method  is  to  apply  a  liquid 
to  neutralise  the  free  lime,  and,  that  object 
being  thoroughly  effected,  the  paint  might 
be  ordinary  white-lead,  although  much 
better  results  would  be  obtained  by  the 
use  of  one  of  the  special  "  concrete  paints  'r 
already  referred  to. 

Among  the  solutions  that  have  been 
recommended  for  use  under  the  coating  of 
paint  are  the  following  :  For  concrete  floors 
the  use  of  sodium  silicate  has  been  suggested. 
Dr.  M.  Bennett  Blackler  advocates  the  use 
of  copals  made  up  with  a  small  quantity 
of  oil  and  diluted  with  turpentine,  and  he 
expresses  his  opinion,  also,  that  casein 
solutions  treated  when  dry  with  formaldehyde 
would  give  a  coat  which  is  absolutely  im- 
permeable to  water  and  form  a  surface 
on  which  paint  would  stand.  In  these 
opinions  Dr.  Blackler  has  been  backed  up 
by  many  other  eminent  men ;  in  fact,  it  is 
not  improbable  that  casein  forms  the  active 
base  of  many  of  the  specialities  now  on 
the  market. 

Assuming  that  the  surface  is  in  as  good1 
a  condition  to  receive  the  paint  as  can  be 


ARCHITECTURAL   AND    SURFACE    TREATMENT 


255 


expected,  the  next  question  arises  as  to 
what  is  the  best  paint  to  apply.  Many 
people  believe  that  when  the  walls  are  in 
a  proper  condition  the  ordinary  paint  can 
be  used  without  difficulty,  and  the  following 
formula  has  been  published : — For  the 
priming  coat :  100  Ib.  of  pure  white-lead, 
9  gals,  of  pure  boiled  linseed  oil,  and  1  gal. 
of  turpentine.  This  may  be  followed  by 
using  instead  of  the  boiled  oil  9  gals,  of 
raw  linseed,  and  1J  pints  of  turpentine 
drier.  Another  priming  coat  recommended 
consists  of  85  Ib.  of  pure  dry  red-lead  mixed 
with  1  gal.  of  boiled  linseed  oil,  and  \  gal. 
of  turpentine.  The  body  coat  should  in 
this  case  be  100  Ib.  of  pure  white-lead, 
4  gals,  of  pure  linseed  oil  (one  third  boiled 
and  two-thirds  raw),  and  the  finishing  coat 
100  Ib.  pure  white-lead,  3|  gals,  of  pure 
linseed  oil  (one-third  boiled,  and  two-thirds 
raw),  and  1  pint  of  pure  turpentine.  The 
red -lead  in  the  priming  coat  above  mentioned 
would  probably  be  in  most  cases  objection- 
able because  of  its  colour. 

When  the  surface  of  concrete  is  covered 
with  cement,  the  latter  may  be  painted  by 
giving  at  first  two  coats  of  Alabastine,  pro- 
ceeding with  oil  paint  in  the  ordinary  way. 
W.  G.  Scott  recommends  as  a  paint  for 
cement  the  follow  ing  :  10  Ib.  plaster -of  - 
paris,  2  Ib.  portland  cement,  8  Ib.  whiting. 
4  Ib.  wheat  flour,  6  Ib.  zinc  oxide,  and  2  Ib. 
red-lead.  This  mixture  should  be  ground 
in  1  gal.  of  prepared  wood  oil,  and  J  gal. 
of  soya  bean  oil.  The  pigments  are  mixed 
separately  with  the  oil,  adding  them  in  the 
order  given,  and  if  the  paste  is  then  found 
to  be  too  stiff  a  little  more  of  the  bean  oil 
is  added.  This  paste,  Mr.  Scott  says,  grinds 
with  some  difficulty,  and  it  is  better  to  mull 
or  run  through  an  iron  mill  first,  and  then 
to  grind  fine  in  a  stone  mill.  In  use,  the 
paint  is  thinned  with  benzine  or  turpentine, 
or  with  a  mixture  of  7  parts  of  turpentine 
and  1  of  paraffin  oil.  The  prepared  wood 
oil  referred  to  may  be  made  by  stirring 
9  Ib.  litharge  and  1  Ib.  of  manganese  borate 
into  50  gals,  of  Chinese  wood  oil,  and  heating 
for  five  hours  at  a  temperature  of  350°  F. 
(177°  C.).  After  the  litharge  and  manganese 
borate  have  settled  out  and  the  oil  is  clear, 
it  is  ready  for  use.  This  paint  or  cement  is 
intended  for  application  to  porous  cement, 


and  may  be  used  on  concrete.  The  same 
author  gives  the  composition  of  a  marine 
paint  which  will  set  under  water,  and  which 
may  be  quoted  here :  10  Ib.  portland 
cement,  5  Ib.  silicate,  3£lb.  zinc  oxide,  7J  Ib. 
red-lead,  5  Ib.  litharge,  and  2  Ib.  graphite. 
These  are  mixed  with  1  gal.  of  boiled  linseed 
oil  and  ^  gal.  of  paraffin  oil.  It  sets  under 
water,  and  becomes  in  time  as  hard  as 
stone.  The  same  mixture  if  thickened  with 
whiting  or  plaster-of-paris  forms  a  valuable 
cement  for  many  purposes. 

Some  careful  experiments  conducted  by 
Mr.   Henry   Gardner,   assistant   director   of 
the  Institute  of  Industrial  Research,  Wash- 
ington,  D.C.,   are  reported  in   his   "  Paint 
Technology    and    Tests."     He    carried    out 
his  tests  on  mortar  made  of  1  part  of  port- 
land   cement,   and  3  parts  of  sharp  clean 
sand.     Omitting    the    failures,  the    results 
were   as  follow  : — No.   1.   Concrete  primed 
with  a  25  per  cent,  solution  of  zinc  sulphate 
crystals  dissolved  in  water.     A  wide  brush 
was  used  for  the  application,  and  the  spread- 
ing rate  was  approximately  200  square  feet 
per   gallon.     Second   and   third   coated   on 
the   second   day   with  the   following   com- 
position :    Sublimed  white-lead  (similar   to 
"  Purex "  in  Great   Britain)  50  per  cent., 
zinc  oxide  35  per  cent.,  silica  and  baryte& 
12  per  cent,  and  Prussian  blue  3  per  cent., 
ground  in  linseed  oil,  turpentine  and  drier. 
This  panel  after  three  years'  exposure  was 
in   good    condition,  although   there   was   a 
slight  checking.     In  test  No.  2  the  concrete 
was  treated  with  a  20  per  cent,  solution  of 
alum  (aluminium  sulphate),  and  a  paint  of 
the     same     composition     as     before.     The 
results  were  almost  identical  with  No.   1. 
In  other  tests  the  concrete  was  primed  with 
a  solution  made  by  dissolving   10  parts  of 
sodium  oxalate  in  100  parts  of  water.     The 
second   and   third   coats   were   linseed    oil 
paints  in  red,  brown,  blue  and  green.     The 
results  are  reported  as  being  "  very  good." 
But  even  better  results  were  obtained  when 
the  second  and  third  coats  were  composed 
of  zinc  oxide  and  barytes  ground  in  an  oil 
(kind  not  given)  having   "  a  low  saponifica- 
tion  value."     This  paint  dried  very  slowly, 
but  the  results  are  reported  as  "  excellent,, 
extremely   glossy    waterproof    surface    pre- 
sented." 


Durability   of   Reinforced  Concrete 


WHEN,  recently,  a  reinforced  concrete  house, 
built  fifty-eight  years  before  at  St.  Denis, 
France,  by  Frangois  Coignet,  was  examined 
and  the  work  cut  away  to  disclose  the  in- 
ternal condition,  both  steel  and-  concrete 
were  found  to  be  in  an  excellent  state  of 
preservation.  It  is  a  practical  illustration 
of  this  kind,  vouched  for  by  a  deputation 
from  the  (British)  Concrete  Institute,  that 
carries  conviction  to  the  minds  of  those 
engineers  who  are  unmoved  by  mere  theories 
and  by  hundreds  of  laboratory  experiments. 
That  concrete  has  an  effective  life  of  hun- 
dreds of  years  under  favourable  conditions 
is  an  established  fact ;  further,  those  who 
have  made  a  special  study  of  reinforced  con- 
crete know  that  in  ordinary  circumstances 
the  steel  will  last  as  long  as  the  concrete  in 
which  it  is  embedded.  The  concrete  exer- 
cises a  protective  and  preservative  action 
on  the  steel ;  and  while  he  concrete  gains 
in  strength  a  hundredfold,  the  steel  gains  in 
durability  a  thousandfold. 

MECHANICAL    DESTRUCTIVE 
INFLUENCES 

Blows  and  Shocks. — Concrete  is  not  an 
ideal  material  to  employ  in  situations  where 
heavy  direct  blows  are  likely  to  fall  upon  it. 
Obviously,  its  brittleness  is  its  greatest 
defect,  and  it  therefore  follows  that  while 
it  makes  a  durable  and  wellnigh  eve